2. The page criteria allows you to target any nodes you want so you can do that very easy.
3. Yes, they can upload their own banners.
If you want to sell ads per category/forum you have to create a package for every category or forum you want (using package page criteria). This way you can set a different price based on category/forum.
4. You can do that by changing the ad template. I think I will add this feature in my next release.
5. It doesn't support this by default but you can achieve that very easy by adding a new hook to the template and create new position(s) for the post(s) you want. It's very easy and documentation is provided.
With this method you can place ads after post x, y, z of every thread and you can use the page criteria to target specific nodes.