WLB Hi William,
WLB 1) If a client wants to be the only person able to upload photos (etc.), is that possible?
I am currently working on a "multi-user" feature, where you would be able to have public version as default, with an option to login for users with "edit" capabilities. This will be released within a few months.
Until then, there is a fairly simple workaround to achieve this:
https://forum.files.gallery/d/223-more-logins-with-different-rights/2
WLB 2) Is the php file the only file to download? Or is there more in a licensed version (.css, etc.)? This question is more for curiosity; the other two are essential.
This is essentially the only file you need to download yes. There is no "licensed version", as your instance will become licensed as soon as you add a license key to the config. Keep in mind, Files Gallery does load several CSS and Javascript files from external CDN (for convenience). If you want to host all the files locally, see self-hosted assets.
WLB 3) How customizable is it? I particularly want to be able to add a top navbar, bottom footer and miscellaneous minor things.
It's not designed to be easily customizable. Not because I don't want it to be, but because the interface is very crowded, especially on mobile devices, and the existing design has been planned meticulously. However, some users have added their own topbar with logo/text, as for instance in this post.
As for modifying the existing features, there is quite a lot that you can do with some custom CSS.