Things seem to work best when plots are relatively independent of one another. In a medium like the muck, where people with vastly different schedules play together, it may be better to think of plots in terms of the relationships between characters ("next time I see X, I'm gonna do Y to them with a Z - assuming their player has time") rather than story arcs. Story arcs are fine in a general way ("my character wants to visit Star City" or "my Strangeified character is going to be unwillingly detoxed by the SMC")... just avoid leaning too heavily on particular other players.
It's always good to make it plain what you want out of your scenes and plots - not in a selfish "gimme this and gimme that" way, but rather a bit of brainstorming and negotiation, if necessary, so that everyone involved has an idea of everyone else's expectations of the plot, and is getting something out of the game.
(Some of the best scenes do come from pure inspiration, but other great ones come from several rounds of OOC "What if..." going back and forth. Interesting things happen when the whole shape of the scene is clear beforehand, like master jazzmen all kicking around an old standard.)
It's also all right to say a polite "no" to a proposed plot development. By the same token, don't take it personally if someone does turn down a plot offer.
It may also be better not to post logs publicly, no matter clever they are and how proud you are of them - unless your plot is very, very public, and you do have time to include anyone who wants to join. Perhaps after a suitable interval has passed? Or with a 'for display only' notice?
How much emotional investment is too much, given that some play on Puzzlebox is coming from deep inside? When you can't think straight, it's time for a break, that much is certain.