About your first point, I'll make a test (asap) with a standard theme to give you a feedback. Anyway I don't remember if I have specified a custom image per page (or news per page) number in my theme. The difference, in the number of pages listed, is the same I get between an admin and a regolar user, so may be the plugin is counting pages with admin privileges (only for the 2 sitemaps I listed above). I have not the same issue for album and image sitemaps.
About your second point, the plugin is using the `root/` path to list gallery pages, while it should use the `root/gallery/` path. The number `5` is an example. I have 5 gallery pages when logged in, and just 2 when not logged in.