Almost everything is doable. THe only problem I see is with the login, but what you require is easy.
Main site point to the forum (and please take a look at extern.php which has functions that you can use to already provide some content), and I'd suggest also a login page (if you don't already have a membership system, in which case you'd have to make them both compatible, which is also doable but requires extra work).
Sub-pages/sites point to each section with a link very much like:
where the last number is the id of the forum, check on the DB or do some trial & error
If the user has already logged in, he'll have the appropiate rights, otherwise, he'll have guest rights, however you've defined them.