Yes, it is very easy to add or remove images from an existing gallery.
You can open a gallery with JuiceboxBuilder (please see here for an overview of JuiceboxBuilder) and you can add or remove image on the 'Images' tab.

After editing a gallery, you'll need to re-upload the gallery's 'config.xml' configuration file (which lists the images to display in the gallery and also stores the configuration options to be used) and also any new images and thumbnails (to the gallery's 'images' and 'thumbs' folders). There is no need to change the gallery's embedding code (in your gallery's HTML page).

JuiceboxBuilder is able to extract the IPTC Document Title (for use as the image's title) and the IPTC Description (for use as the image's caption).
Once the data is extracted from the images, the image title and image caption are displayed in the gallery's caption area.
The only way to extract any other EXIF or IPTC metadata would be to use the Juicebox plugin for Lightroom to create your galleries. (Adobe Lightroom would be required.)
Lightroom has the ability to extract whatever metadata you like from an image (EXIF or IPTC) and the plugin can make use of this for the image titles and captions.
Once the plugin has been installed, you can select the Juicebox web layout, scroll down to the 'Image Info' section, choose 'Edit...' from the drop-down menu (for the image titles or captions) and you'll find the functionality for extracting metadata there. You can build up an image title or caption using whatever custom text and metadata you like.
More details on the Juicebox plugin for Lightroom can be found on the plugin's support page here.

All of the above (adding and removing images using JuiceboxBuilder, uploading the updated 'config.xml' file and extracting EXIF and IPTC data using the Juicebox plugin for Lightroom) are true for both Juicebox-Lite and Juicebox-Pro so you could certainly try things out with Juicebox-Lite before purchasing Juicebox-Pro if you like.
You can download Juicebox-Lite from the download page here.
Be sure to check out our Getting Started guide which will guide you through the process of creating your first gallery with JuiceboxBuilder.

You're welcome!

It is not possible to open an existing gallery in Lightroom so you'd have to recreate the gallery from scratch.
However, this is not as difficult or time consuming as it might sound.
You can save a gallery's images as a Lightroom 'Collection' and you can save a gallery's configuration options as a Lightroom 'Template', like a 'Preset' in JuiceboxBuilder (please see this forum post for details for details).
With this in mind, you can quickly open a 'Collection' of images, apply a 'Template' and then just edit the gallery as required (for example to add or remove images) before exporting it to your hard drive.

In case you are interested, there is actually a Juicebox skin available for jAlbum which allows you to create a multi-gallery website. You can use a complete folder structure as the album's input and the skin will create a Juicebox gallery for each and every folder within the structure and link all the galleries together via a navigation tree in a side menu.
The Juicebox skin for jAlbum can be found here (and comes bundled with Juicebox-Lite, the free version).

With Juicebox-Pro, you can create as many individual galleries as you like. However, if you would like to link them all together (with a navigation menu), then you would need to do so manually following the examples in the Embedding Multiple Galleries support section (or by using the Juicebox skin for jAlbum).

If you are looking for an automated solution to creating and listing multiple galleries, then you might be interested in another one of our products, namely Showkase.

Showkase is a PHP web application (installed on your web server instead of your computer) which allows you to create a complete portfolio web site (integrating multiple galleries) online.
Showkase has full support for Juicebox-Pro and the galleries can be created within the application itself in a web browser interface (or created with JuiceboxBuilder or the Juicebox plugin for Lightroom and imported).

You can create Gallery Index pages and have as many galleries listed on each Gallery Index page as you wish.
Each gallery is represented by a thumbnail image with the gallery title displayed below and the gallery is opened when the user clicks on the image.
Demo sites created with Showkase can be found here and a sample Gallery Index page can be found here.

Showkase can also create non-gallery pages (About, Basic and Contact pages) where you can add information about yourself (or any other content you like).
All of this is done automatically within the Showkase interface without the need for any manual coding at all.

Showkase can be purchased as Showkase-Standard (which comes with Juicebox-Lite, the free version) or Showkase-Pro (which comes with Juicebox-Pro).
The only difference between Showkase-Standard and Showkase-Pro is the bundled viewer (Juicebox-Lite vs Juicebox-Pro).

More information on the differences between Juicebox and Showkase can be found in this forum post.

I hope this helps.