Yeah, I know how that is. Keep trying and you'll get better, no doubt about that. If there is a way that will ensure activity in and of itself, I haven't seen it. Best you can do is create something of reasonable quality and seek the best people for the job. I like you idea of having a dedicated game master, but I also think that having a group of dedicated group of core players is important so that things keep moving no matter what, in your idea and any other, really. Flexible with other players, but keeping that core there so things don't go completely off the rails or up and die. Roleplays live and die by the players, ultimately, so that's what I'd focus on. I hope I'm being at least somewhat helpful, I really do.
Hm. That's kinda one of those situation where it's like you want to have your cake and eat it too. I'd suggest a time-sensitive turn system of some kind to be established from the beginning so control can be effectively separated, but that would also undermine the desired consistent flow and interest, at least a little. As for rules to be followed, my mind goes to character traits and limitations, maybe even a true stat system, depending on how heavy you want to go on the RPG elements. I'd definitely agree that at least a basic dice roll system would be in order if you want to go that way. These things combined, you got a fairly solid base for a consistent, single-character roleplay, tabletop RPG-esque experience, in theory. Seems like a matter of balancing rules and improv abilities at the heart of things, balancing the subjectivity of multiple players and objectivity of a single character. Maybe my perspective is just totally off.
It's a strange thing to talk about in theoreticals and I'd say that it would be best to start something like you're describing as soon as possible to see how it goes in execution. Learn by doing and keep trying until it works. That is about the only formula I've seen work in practice, but someone else might know better than me.
I didn't intend to talk this much or this long. I'll stop now.