1: You could use
Category Permissions to limit uploads to one file per user in a certain category.
2: You can set the Category Number in the PhotoPlog
Settings, but it would be applied to all categories. Also, you could remove the links to the view file page/view original image via template edits, but if a user knows the links, the user could still access, and view, the page or image.
3: This would require custom code, but please note that coding custom modifications is really outside the realm of support, so I won't provide custom modifications. Sorry.
4: Same answer as three.