You need to declare $forums_that_are_categories as a global within your two filter functions. Other than that, it works great.
It’s true! Trunk does 
“would you be able to go into a category and see posts from all the sub-forums like a latest discussions list but limited to those few forums?”
Yes, as well as a listing of forums in that category.
There was a large commit to trunk a few days ago that upgraded most of the cookie stuff.
WP/bbPress session integration only works if the installs are on the same domain/subdomain. This is limitation of cookies, and a good thing. Not a wp/bb issue.
From the docs:
Cookies
Integrating cookies allows your users to log in to either your WordPress blog or your bbPress forums and be effectively logged in to both. This feature requires your WordPress and bbPress installations to exist in the same domain name, or at least in sub-domains of the same domain name.
Take a look at:
https://bbpress.org/documentation/integration-with-wordpress/
