Let me help get this going by adding what I know from building the bbPress Topics for Posts plugin:
1) Yes, it works perfectly even without BuddyPress.
2) Having a post on one site and the forum on another will probably NOT work. Forums and topics are considered posts themselves, and those are per-blog. Anything is possible, but that would be messy.
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5) This isn’t too hard, but takes a little trickery with the bbp_has_replies_query filter. It’s a feature of the plugin if you want to take a look to see how it’s done.
