Well, if someone wants to do (3) and pin his post for some time, you can just unpin (1) temporarily during the period when his post is pinned. That way, you will only have two pinned posts at any point in time.

I think rotating pinned posts might help because I imagine a flurry of contributions once (1) is available. But at some point in time, the thread will become dormant since I doubt we are that prolific haha. It will be quite boring to read a pinned post with hardly any evolvement all the time