It seems strange that activating WP-Juicebox would prevent your web site from triggering its mobile mode.
Usually, factors such as the device (browser user agent), screen size and screen resolution will be taken into account when deciding whether to use desktop vs mobile mode but, from what you say, your theme's logic seems to also be considering page content. Maybe your theme's author can help determine why your mobile mode is not being used when WP-Juicebox is active. Other than loading the 'juicebox.js' JavaScript file and having a <script> tag with the gallery's embedding code, all that WP-Juicebox inserts into the web page is a <div> container for the gallery.
Checking out your website in Mobile Safari (iOS 10.3.1), the big difference between gallery pages and other pages seems to be that most gallery pages have large text at the top (above the header) such as "North Hollywood, Malibu, CA: Iron Gates in Los Angeles" in the link you provided. It seems to be this text that is not being resized appropriately. The rest of the web page seems to be responsive (the header fits the width of the web page and the navigation menu is hidden under the icon).
It might just be that your theme is not taking this text into account when resizing content for mobile devices.
The "Error: No p element in scope but a p end tag seen." validation message seems to be coming from the following two sections of code surrounding the gallery's embedding code.
<p>[vc_row el_position=”first last”][vc_column][vc_tabs interval=”0″ el_position=”first last”][vc_tab title=”Pictures” tab_id=”1357613662131-6-0″][vc_column_text]
[/vc_column_text][vc_column_text el_position=”first last”][/vc_column_text][/vc_tab][vc_tab title=”Glass Information” tab_id=”1368221702106-4-9″][vc_column_text el_position=”first last”]</p>
If you view the source of your web page in your browser, you'll see this code surrounding the gallery's embedding code.
The plugin only inserts the code between <!--START JUICEBOX EMBED--> and <!--END JUICEBOX EMBED-->. The code I posted above (including the <p> and </p> tags) are not inserted by WP-Juicebox.
WordPress can sometimes insert unwanted <p> (and <br />) tags but usually only when you enter HTML code directly into the WordPress editor (in 'Text' mode). (Maybe you've got some code other than the WP-Juicebox gallery shortcode in your editor.) This is usually due to WordPress's wpautop function.
You could try installing a third-party plugin to disable the wpautop functionality, for example Toggle wpautop or wpautop control.
Otherwise, you could perhaps implement the manual solution from this forum thread.
Alternatively, you could try installing the Raw HTML plugin and wrap any HTML code that you have in the WordPress editor in [raw] ... [/raw] tags. This should prevent extra markup (such as <p> and <br /> tags) from being added to the HTML code by WordPress itself or any third-party theme or plugins.
This might help with the <p> </p> tags but I do not know where the [vc_*] tags are coming from. They look like unprocessed shortcode tags (but I do not know their origin).
Incidentally, it looks like your theme's custom CSS is affecting the layout of your WP-Juicebox galleries.
When I view a gallery on a mobile device, the Splash Page image is offset (to the top left). This is more than likely to be due to some CSS which is being applied to all images on your web page (including those in your gallery).
Specifically, on line 1 of your 57901af0a3c58.css file, there is the following code:
img, video, .wp-caption {
-moz-box-sizing: border-box;
-webkit-box-sizing: border-box;
box-sizing: border-box;
max-width: 100%;
height: auto !important;
}
These CSS rules are being applied to all images (<img> tags) on your web page and the WP-Juicebox gallery has no option but to inherit them. Ideally, those CSS rules should be applied to only those elements on your web page which require them through use of CSS selectors (classes or ids).
I hope these notes point you in the right direction.