This is insightful and pretty indepth conversation on the topic.  I'm gunna just put in my 50 cents in a simpler fashion.  

Would it be viable for every player to be a GM in every story on this site at The Same Time Through The Whole Story?  In my opinion no because it's too confusing about who's deciding where the next destination is at a given post and because it would take too much collaboration of people rapid posting back and forth to get a single post out.  People's timezones compounds the problem.  

Now I think your idea can be tweaked and could work in a story within the site.  There was a roleplay I was a part of a long time ago.  Where one gm would have control through one story arc and it sort of passed to the next player for the next story arc.  Now that's for a single story if you and the other players want to do that.