I’ve edited your topic title to be more specific to your post.
Your original title – “doubts about buddypress” – is not even related to what you have written.
re: filtering the activity stream – unless I’m mistaken, it’s not possible to tack on multiple scopes or user ids to the activity stream loop.
re: single-level threaded stream – I’ve thought about this myself. I’ll look into this when I have some time.