Thank you for posting the content of your XML file. I see the problem now.
The numerically-named XML file in the 'wp-content/uploads/juicebox' folder is not the XML file that Juicebox uses when the gallery is displayed. It simply stores the configuration options for the gallery (no image data) and other plugin settings.
Galleries created by WP-Juicebox do not use static XML files (like those generated by JuiceboxBuilder-Pro).
The gallery's actual XML data is generated dynamically at the time the gallery is displayed (from the 'config.php' file and functions within the 'wp-juicebox.php' file) using the options and settings stored in the numerically-named XML file.
It is not possible to add image data to the numerically-named XML file (and trying to adapt the plugin to display custom images would not be easy).
The plugin will always display all images attached to the post.
To display other images, you could either:
(1) Use Juicebox-Pro's Flickr options to display images from a Flickr account.
(2) Create a gallery on your computer with JuiceboxBuilder-Pro and embed the gallery manually in your post following the baseUrl method of embedding documented here (which I thought you might have initially been doing).
(1) Enter your gallery's baseUrl embedding code into a 'Formatting -> Custom HTML' Gutenberg block.
(2) Upload your complete gallery folder to your root directory and use a leading slash in your embedding code paths to denote your root directory. This should work with any permalink structure. For example:
<!--START JUICEBOX EMBED-->
<script src="/gallery_folder/jbcore/juicebox.js"></script>
<script>
new juicebox({
baseUrl: "/gallery_folder/",
containerId: "juicebox-container",
galleryWidth: "100%",
galleryHeight: "600",
backgroundColor: "#222222"
});
</script>
<div id="juicebox-container"></div>
<!--END JUICEBOX EMBED-->You can upload your gallery folder to any directory on your web server as long as the two paths in the embedding code (the path to the 'juicebox.js' and the baseUrl itself, pointing towards the gallery folder) are correct. (Your root directory might just be a convenient place for you to upload your gallery folder.)
I hope this helps.