Fellowship Adventures are 1 of the *BIG* reasons I keep mages and ambassadors restricted to experienced, trusted, long-term members of the FS. Have you ever come up with a plan for a FA, communicated that plan to the fellowship, and then had the entire plan disrupted because an overeager ambassador pulled the stage 1 or stage 2 flag early? I sure have! I will say I have never considered the approach you seem to be talking about, temporarily demoting your ambassadors to fellows for the week. I'll have to consider that.
I for one like when I'm demoted, along with everyone else (as long as I am returned to my normal status after the FA). There's a peace of mind in knowing that the flag won't be pulled by some id--- um, I mean, by some member. I've heard all the excuses. We've had the flag pulled early at least five times since I've been doing FAs, in several different fellowships. The worst part is, it isn't usually the newbies that do it. It's the players you assume have more sense than that. One said that she didn't realize she was pulling the flag. She had more experience points than I did, and I was no newbie. Another said that he was sorry, but he let his 12 year old play for him for just a few minutes. Another actually ... and I kid you not ... blamed his cat for jumping on the keyboard at precisely the moment he was looking at the prizes. What are you supposed to say to that? I know what I'd like to say, but I always wind up with, "aw, that's too bad" or some lame thing that makes them feel better. It may only be a temporary frustration, but it is real and very annoying at the time, particularly if you've only got one more item to put in a 1500 way point when that happens. (Yes, that's happened too.)
I have no problem whatsoever being demoted. I am glad to know that the archmage is taking care of all of us, to ensure that no one lessens the enjoyment of the whole group or causes any hard feelings between members. Whenever I run FAs for the fellowship, I do that myself, and make sure everyone knows that it is only to avoid accidents and to protect the members. I've designated one or two other people as flag pullers (making sure they were on different schedules from me so that no one had to wait too long to get it pulled) and everyone understood and were happy with it. The experienced players were usually happiest of all, having known a time or two when they had known that frustration.
I know some archmages say "well if you communicate well you shouldn't have to do that," but after you've seen it happen even in the best of fellowships, you know that all the communication in the world doesn't stop a cat from jumping on a keyboard (okay, I know it was probably bogus, but that was a high level player who heard all the instructions. What hope does an AM have if even the big guns do that?) Anyway, that's my 2 cents.