Actually this isn't covered in the FAQ directly.
I think you're a bit confused. WordPress the software that you get from WordPress.org and WordPress.com are two different beasts, gg. We run here a software called WordPress MultiUser. You can find a list of differences here.
But to answer your questions:
1 + 3) No as this is not normal hosting. The software just gives you a blog to post at with no access to the actual files. You best bet for a forum would be to create one at one of the free forum hosting sites and link to it directly. Another solution would be to obtain hosting elsewhere, install the WordPress software there, and obtain a forum and install that. A good option would be to obtain bbpress as it's made to work along side WordPress. (You can get them to share the user table with a single line)
2) That would be actually downgrading as WordPress MultiUser is basically running 2.1 code and features for the most part.
But, as TT suggests, take some time to read over the FAQs. That will probably help as well.
Hope this helps,