If you are trying to add a custom caption to just one image using the Juicebox Plugin for Lightroom, then do the following:

(1) Click the 'Library' module tab.
(2) Select the image you want to add the caption to.
(3) In the 'Metadata' section, add the following to the 'Caption' box (changing the link and/or text as appropriate):

<a href="https://www.roatan.photos/credits.html">Click here to go to 'credits' page.</a>

(4) Click the 'Web' module tab.
(5) Select 'Juicebox-Pro' as the 'Layout Style'.
(6) Scroll down to the 'Image Info' section and make sure that the 'Caption' is set to {Caption»}. (This is the default value so you should not need to change anything.)
(7) Save your gallery to your hard drive via the 'Export...' button.

Step #3 embeds the caption as the IPTC Caption within the image.
Step #6 uses each image's IPTC Caption as the gallery's image caption. (Images without an IPTC caption will simply have a blank/empty caption.)

Alternatively, you could either:

(1) Open your gallery in JuiceboxBuilder-Pro (after creating it in Lightroom), click the appropriate thumbnail on the 'Images' tab and enter the caption (below) in the 'Caption' input box before re-saving the gallery on the 'Publish' tab.

<a href="https://www.roatan.photos/credits.html">Click here to go to 'credits' page.</a>

... or:

(2) Edit your gallery's 'config.xml' file in a plain text editor (again, after creating it in Lightroom) and replace the image's <caption> tag manually, e.g.:

<caption><![CDATA[<a href="https://www.roatan.photos/credits.html">Click here to go to 'credits' page.</a>]]></caption>

I hope my notes above help.

In order for captions to be visible, you'll need to set captionPosition to something other than NONE (in JuiceboxBuilder-Pro's 'Customize -> Caption' section).

If you do not want a separate area to be reserved for captions within your gallery (which would be empty for images with no caption text), then set captionPosition to either OVERLAY (where the caption area will overlay the bottom of the gallery's image area, spanning the width of the entire stage) or OVERLAY_IMAGE (where the caption area will overlay the bottom of the main image, spanning only the width of the main image itself).

Also, if you want the caption area to always be visible (rather than autohide after a set period of inactivity determined by the value of the inactivityTimeout configuration option), then set showImageOverlay to ALWAYS (in JuiceboxBuilder-Pro's 'Customize -> Main Image' section).

Your captions are malformed.
Each <a> tag needs to have a closing </a> tag (after the text which is to be used as the link).
For example:

<a href="http://www.example.com">Link text goes here</a>

Also, imageClickMode="OPEN_URL" is a Juicebox-Pro configuration option (which can be set in JuiceboxBuilder-Pro's 'Customize -> Main Image' section) rather than an attribute for an HTML <a> tag.
(If you want to set imageClickMode="OPEN_URL" manually by editing your gallery's 'config.xml' file in a plain text editor, then you should add the configuration option as an attribute to the opening <juiceboxgallery> tag alongside the other configuration options).

Here's a sample caption that you could add to an image on JuiceboxBuilder-Pro's 'Images' tab which would display some sample text with only the word 'here' being a clickable link.

Click <a href="http://www.example.com">here</a> to go to another web page.

I hope this helps.

There are essentially two ways to make a Juicebox gallery responsive (with its dimensions dynamically changing with the size of the user's browser window).

Scenario #1:
A Juicebox gallery will be responsive (and will dynamically scale with the size of the user's browser window) if the gallery's own dimensions and the dimensions of all parent containers are expressed as percentages. If there is a fixed value anywhere up the chain, then the gallery's size will become fixed (e.g. 100% x 100% x 800px = 800px).
Please note that when using percentage heights, you may need to implement the suggestion noted here.

Scenario #2:
You could use JavaScript to listen for a change in the size of the user's browser window and assign new dimensions to the Juicebox gallery if and when this happens.
An example of this can be found in the resizable galleries support section here.
Take a look at the source of this sample gallery in your browser to see how this might be achieved.

If you leave galleryHeight empty, then Juicebox will use the default value of 100% (so leaving galleryHeight empty is the same as setting galleryHeight to 100%).

If setting galleryHeight to 85% works within your own web page, then that is absolutely fine.

If you set captionPosition to BELOW_IMAGE, BOTTOM or BELOW_THUMBS, then the height of the caption area will be fixed at the maxCaptionHeight value (default value 120px). (If you set captionPosition to OVERLAY or OVERLAY_IMAGE, then the height of the caption area will be variable up to the maximum value set via maxCaptionHeight.)
You can change the value for maxCaptionHeight in JuiceboxBuilder-Pro's 'Customize -> Caption' section.

Yes. Set showImageOverlay="ALWAYS" in JuiceboxBuilder-Pro's 'Customize -> Main Image' section.

I hope my notes above help.

Your gallery sets showThumbsOnLoad="FALSE" but also sets autoHideThumbs="TRUE" so, although the thumbnail area is not displayed when the gallery is initially displayed, itis displayed when you hover over the gallery (but you also 'hide' the thumbnail images by setting maxThumbColumns="0", maxThumbRows="TRUE", thumbHeight="0" and thumbWidth="0" so only the thumbnail navigation arrows are visible).

All you need to do to disable the thumbnail area completely (and thus hide the thumbnail images and navigation arrows) is set showThumbsOnLoad="FALSE" in conjunction with autoHideThumbs="FALSE" (its default value) so that the thumbnails do not reappear on hovering over the gallery.

There are a few ways to include links within a Juicebox-Pro gallery.

You can assign a unique link (known as a linkURL) to each image in a gallery. (You can add individual linkURLs on JuiceboxBuilder-Pro's 'Images' tab).

Normally, when the 'Open Image' button (in the gallery's Button Bar) is clicked, the image itself is opened.
However, if an image has been assigned a linkURL, then the linkURL will be opened instead (in a new browser tab or the same tab, defined by the corresponding linkTarget).

Also, if you set imageClickMode="OPEN_URL" (in JuiceboxBuilder-Pro's 'Customize -> Main Image' section), then the linkURL will be opened when the main image itself is clicked.

A linkURL can be a relative URL (relative to the web page containing the gallery) or an absolute URL (in the form http://www.example.com/index.html).

Alternatively, you can include a link in an image's title or caption by using HTML formatting as noted in this FAQ:
How do I add HTML formatting to image captions and titles?

Check out the link to our home page (via an image caption) in the first image in this demo gallery.

If you want to include a link in only one image in your gallery, then it might be best to do so via an image caption rather than by using linkURLs and setting imageClickMode="OPEN_URL" (as you might not want other images to have active links) but I hope that my notes above give you some ideas as to how you might like to proceed.

@jasonsmith

I'm glad my suggestion helped you out.
Thank you for letting me know.

It sounds like your problem might be server-specific.
Do you see any flickering when you view this test gallery (on our own server): https://www.juicebox.net/demos/pro/full/
I do not see any flickering when I view this test gallery and navigate between thumbnail pages or image pages on my own iOS 12.3 device.

Check your web server's cache control settings.
It sounds like your web server may be set to prevent browser caching which might account for the problem you describe.

Also check your gallery's embedding page for any meta tags which might prevent browser caching.

I hope this points you in the right direction.

I'm glad it worked! Thank you for letting me know.

The "Front" of the gallery that you describe is the Splash Page which is displayed by default on small screen devices when the gallery is embedded in a web page alongside other content.
When the Splash Page is tapped, the gallery is expanded to fill the browser viewport (giving the images more room to be displayed).
Please see the Screen Modes section of the Gallery Tour for more information on the Splash Page.

By default, the Splash Page uses the first image in the gallery.
In your case, the image is offset (and not filling the Splash Page container as expected) because of some custom CSS on your gallery's embedding page.
On line 665 of your 'style.css' file ('/wordpress/wp-content/plugins/jetpack/modules/minileven/theme/pub/minileven/style.css') is the following code:

/* Images */
.entry-content img,
.comment-content img,
.widget img {
    height: auto;
    max-width: 100% !important; /* Fluid images for posts, comments, and widgets */
}

The max-width CSS rule is being applied to all images (all HTML <img> tags) within the specified CSS classes and this includes the Splash Page image. The Splash Page image has no option but to inherit this custom CSS rule.
Removing the !important declaration from the end of this custom CSS rule should be enough to reposition the Splash Page image correctly but, if other images on your web page require the !important declaration , then the best course of action would be to apply the max-width CSS rule to only those images on your web pages that require it through use of further CSS selectors (ids or classes).

Incidentally, you can choose to not use the Splash Page if you like by setting showSplashPage="NEVER".
If you continue to use the Splash Page (recommended for small screen devices), then you can configure the Splash Page (for example by choosing a specific image to be used) via the Splash Page configuration options.

I hope this helps.

There is no interface control to prevent the ALT tags from being used as image titles but you can disable them with a quick edit to the plugin's 'wp-juicebox.php' file.
Open the 'wp-juicebox/wp-juicebox.php' file in a plain text editor and comment out (or remove) lines 977 to 983 inclusive.
Just change:

$title_element = $dom_doc->createElement('title');
$image_title = $attachment->alttext;
$image_title = $this->line_break($image_title);
$image_title = $this->strip_control_characters($image_title);
$title_text = $dom_doc->createCDATASection($image_title);
$title_element->appendChild($title_text);
$image_element->appendChild($title_element);

... to:

// $title_element = $dom_doc->createElement('title');
// $image_title = $attachment->alttext;
// $image_title = $this->line_break($image_title);
// $image_title = $this->strip_control_characters($image_title);
// $title_text = $dom_doc->createCDATASection($image_title);
// $title_element->appendChild($title_text);
// $image_element->appendChild($title_element);

The leading // comments out a line and it will just be ignored rather than parsed and executed.

(The descriptions will still be used as the image captions.)

Please note that the line numbers above refer to the current version of WP-Juicebox (v1.5.1.1).

The default value for showDownloadButton is actually FALSE (i.e. if you do not explicitly set showDownloadButton="TRUE", then the Download Button will not be displayed in the gallery's Button Bar).

The default value for showOpenButton is TRUE but you can disable the Open Image Button by setting showOpenButton="FALSE". (You can set showDownloadButton="FALSE" too, if you like, just to be sure that the Download Button will not be displayed.)

For reference, a list of all configuration options and their default values can be found on the Config Options page.

If you continue to experience difficulties, then please post a link to your gallery's web page so that I can see the gallery for myself. Please let me know what you see and what you expect to see and I'll do my best to help you out.
Please also confirm whether or not you are using the Juicebox Module for Drupal. Thank you.

If you are using the Juicebox Module for Drupal, then first of all, please make sure that you you are using the Juicebox-Pro 'jbcore' folder from the Juicebox-Pro zip package in Step #3 of the installation instructions here.

Afterwards, you should be able to enter Pro configuration options (such as expandInNewPage="TRUE") into the Juicebox Library - Pro / Manual Config section of your Drupal Dashboard (Home -> Administration -> Structure -> Content types -> Article -> Manage Display -> Juicebox settings).

Pro configuration options should be added as text, for example by typing or pasting expandInNewPage="TRUE" in the Juicebox Library - Pro / Manual Config section. (You will not find individual controls for each Pro configuration option within the module's interface.)

Please note that the Juicebox Module for Drupal is an unofficial plugin which was not written by ourselves.
As such, if you are experiencing difficulties with the module and the module's own documentation (found here) does not help, then I would recommend that you post your query in the Drupal forum where the author of the module (Ryan Jacobs) should be able to help you further.
He is active and very helpful in the Drupal forum and has a much better knowledge that I have of both the module and Drupal itself.

Incidentally, an alternative to using the module would be to create a gallery first on your computer using JuiceboxBuilder-Pro and then embed the gallery into your Drupal page using the baseUrl method of embedding following the Embedding in a Drupal Site instructions.

I hope these notes point you in the right direction.

Thank you for the additional information.

From your screenshot, it looks like you may have set imageVAlign="TOP" which aligns the top of the main image with the top of the gallery leaving whatever space remains in the gallery below the main image.
If you want an equal amount of space at the top and bottom of the main image, then set imageVAlign="CENTER" (the default value for this configuration option) in JuiceboxBuilder-Pro's 'Customize -> Main Image' section.

If this does not resolve your problem, then please check the CSS on your gallery's embedding web page to see if you have any custom CSS rules which are affecting the layout of your gallery (for example, any custom CSS rules which apply to all img tags on your web page which the gallery may be inheriting).

I hoe this helps.
However, if you continue to experience difficulties, then please post the URL to your gallery's web page so that I can see the problem for myself and investigate further.
Thank you.