After doing some more work on this, I think that while the visibility of my topic in the subforum is indeed broken after the Subforum’s ID is placed in the _bbp_hidden_forums option, that is only a symptom and not a cause.
I think that the fundamental assumptions of this version of bbPress/BuddyPress preclude the use of Subforums. And evidence of this is that visibility sensitive areas of the code like this just come up with the wrong answer when subforums are involved.
So for me, I think I’m going to stop trying to make subforums work until the core code changes to better support them.
