I personally find closed (usually one-on-one) threads easier mainly due to the issues you raise.
Doesn't stop me trying to do them though.
I love a challenge!
Good luck on this thread and thanks for posing the questions!
i prefer the one on one threads myself just for many of the reasons you have stated. It is difficult to write to a trhead where they leave tyou hanging around doing nothing because te other characters are interacting and there is nothing for you to do, but you are expected to post anyway...leaving these one or two line posts (which is my personal pet peeve)