I think the question you are asking is, how do you not overwhelm a single DM with keeping everything straight in terms of out-of-game preparation. It sounds like your in-game sessions are fine, it is just the getting up to that point that is the overwhelming issue.
I would take a cue from cooperative world building or even the old Sanctuary book series. Set down with the whole group one session and layout a frame for the world you are in (if you haven't already done so). This will give each rotating DM a stable environment that is familiar to the group. Make it as detailed or simple as fits your group.
Note: Do not say what each adventure is about, just what the world is about. (I'm guessing the you have done this to some degree.) You can easily start where ever you are in the game.
Now with a common starting point the storyline can spring board from there. Each session should be in line with the world framework and yet each will move the storyline along in some way.
Use some type of digital recording system so that notes, NPCs, locations, and events are well documented. A Wiki-based system is ideal for this and there are some really good ones to choose from. It can be open to all players but the current D.M. can keep his adventure notes off line until after it is over. During the adventure any player who feels so inclined or a specifically assigned player can enter information on what has happened with each session. When everything is ready to switch DMs, the outgoing one just reviews and fleshes out needed information on the wiki and passes along secret stuff like, ideas, thoughts or even unknown consequences, to the next DM.
I think you will find this gives each incoming DM a kick start in the creativity area, which I have found to be the best way to save time. If your imagination is already running the pregame session prep goes much quicker.