Hi,
Personally, I would wait, especially if it were closer to 12 proposals out and/or this is your first round of proposals out (ie you haven't gotten any feedback, yet). And, yes. If you get an offer, immediately let everyone know. That should speed up their process, they'll read quickly (some might say something like, "Can I have until the weekend?" and it is totally up to you whether you want to grant that, but if the person was higher up on your list than the one who made an offer, why not?), and then you'll see if you're in the lucky position of having a choice. Speaking of which, talk to the agent who made the offer, see what you think and if you still have proposals out with agents you want to consider, be very enthusiastic and appreciative, but also say that you need a little time to make a decision.
I'm not sure what you think is a "problem." You must have a very good query to now have several proposals out. Even IF no one makes an offer on this batch, perhaps a few will give some feedback. Mull it over, decide if it's valid and if so, rewrite the proposal before querying anew.
Really, it sounds like you're very much on the right track.