BP Links is an old, unsupported plugin. The error message is saying “hey, this bit that BuddyPress provides is missing (since you deactivated BuddyPress), and so we can’t load it, so here’s an error”.
For site wide forums, it’s admin menu is in wp-admin as “Forums” item. If you can’t see it, go to your plugins and make sure you have bbPress 2.x activated.
On this site, it ties into the WordPress.org shared user accounts. Probably they just don’t let you change your WordPress.org details; I’ve never had any luck. We should probably remove the option from the profile screen, I’ll feed back.
Unless you are an advanced user, who is able to figure out why things break when they break (this does not mean asking on the forum for support), do not install trunk.
For regular multisite advice, you’re best to post on the WordPress.org multisite forums for expert advice. As far as BuddyPress goes, I’d worry about getting WordPress sorted first, and then seeing what state BuddyPress is in.
