"When it comes to a play-by-post style of game, however, it's harder; I have to sort of "guess", for lack of a better way of putting it, what it is you'll want to react to"
Therein lies the problem. You seem to be so hung up on what other people want, that you're both consciously and subconsciously stemming the natural flow of things. This is true for RPs as well as life. News flash! There's little to nothing you can do to make people react the way you want them to. Even if you're doing everything right, they may react in a way that's completely wrong regardless. The best one can do is to just be honest with how one feels about the situation one finds oneself in. No more can be expected of one. Anti-climactic advice I know but that's all there is to it.
They say life isn't easy. People say this because the difficulty of many scenarios can't be controlled very well. But guess what!? Life isn't meant to be played, it's meant to be lived.
TLDR: Be honest with what you want and screw what people think! If people give you shit, then I guarantee, that they don't have the courage to do the same. Embrace the chaos.
As for the mission length. I say let them play out for as long as it needs to. Until the objective is achieved of course. Getting lost and distracted is fine but a gentle nudge here and there towards the mission objective is warranted.
I'm glad we're all sharing @Archmage MC@Hylozoist. Perhaps maybe we can express these things more through our artistic contributions? That would make for something very interesting. I for one can't wait to go back to the game! n_n