Much like a business, government, project, family, or any sort of team effort, it's usually a lot easier when you have one person who can make all the tough decisions. The reason why people go for the GM/Co-GM format is because it's usually easier to run things that way. But heavy is the head that wears the crown. 

I think that RP players are just a little flaky by nature. Often we have larger than life ideas for our characters and RPs and reality never quite measures up to them. I think you mentioned you wanted to plan your RP before it even started. I think that you're going to need a way to decide who's plans go in and who's don't. Moreover, I've found that the most successful GMs aren't the ones that have a contingency for every hickup, but are good improvisers. They plan well, but are flexible enough to adapt to things that nobody could see coming.

Honestly, a good GM will listen to their players and let them affect the plot's direction. Regardless if it's something very on rails or more of a sandbox environment. 

I can't say for sure if this "everyone's a GM" thing would work or not, but if your issue is that the GM keeps leaving, maybe become the GM yourself? I've almost exclusively GM now and only join RPs by proven GMs and I have few issues with how long the RPs I join last. And as others have said, having a group of people you trust is good too. Just remember that RPs can last years and that's just long enough for life to do some nasty shit to your friend groups.