Play-by-post has a unique set of...I don't want to call them "problems", but maybe "challenges" might be a better word. In a perfect world, everyone would be able to post multiple times a day and we could just zip right through everything. Unfortunately, it's not perfect. Some people, like me, are able to spend the better part of most days derping online and can post with frightening frequency. Other people have other things they enjoy, or that they're obligated to do, others just have...stuff happen. Life, work, illness, whatever. I've actually had two sagas die under me (one twice!) because the storyguide disappeared without a trace. (One of them, it turns out he was battling cancer, but nobody else in the game knew this).
I think the key is keeping the lines of communication open. I think we should all have a somewhat regular (if infrequent) posting schedule, and if something comes up that's going to interfere, a quick "Hey guys, me and the boyfriend are going on vacation for the next week, don't know if we'll be able to post" shouldn't take that long, and saves a lot of stress.
As for me, I think I've got my profile set to show "Last Visited," and I'd be real surprised if it ever showed anything more than 24 hours old
But, anyway, back to the question I think you asked.
I think having the "lead" character works. And I kinda like the investment idea, with qualifications. If another character (magus or companion) is brought in by his player, then that player is making a commitment, in a way. It's like he's saying, "I understand that the lead player has a certain posting rate, and by bringing my character in, I am committing myself to post at a similar rate." If for whatever reason that player isn't able to make that rate, then he's (by bringing his character in) agreed to let the lead player or the SG make decisions on his behalf or run his character for him until he's able to return, with not a real cause for complaint about what happens unless things totally go south. The flip side to this is that the Alpha Player and the Storyguide have a duty to try to keep the character "in character" and not have them do something totally out there.
For myself, I try to have a three-day rule of thumb: if there's something I see that I should respond to, I will do so within about three days. (If I ever go longer, feel free to shoot me a pm, I might have missed it, or seen it right before I had to run Mom to the doctor's office or something and forgotten about it.)
Don't know if this helps any, but I hope it does.