To answer your questions:
Harbaugh wouldn't need to wait as long as most coaches just because where he's at is almost like jumping from HS to CC or something along those lines. He's at a U without a scholarship
FB program and he would be leaving for a BCS program so I don't think the pressure would be on him necessarily to wait past early next week to announce things. Arguments could be made that it's only fair for him to wait but that doesn't mean he couldn't sign the papers and keep it quiet;)
As far as who else is after him, I've heard primarily Michigan State but for some reason one source likes to mention Miami even though I don't see that happening for numerous reasons. Stanford could be but right now they aren't even sure if they'll be able to get rid of their coach and DMac would be a very possible candidate there.
Brewster is still very much a possibility for
ISU, I just have been hearing so much in regards to Harbaugh and talks with
JP that I don't see it happening unless things fall through with JH and
JP. Sources have been very adamant that it will be Harbaugh and they're doing a good one on me if it isn't looking like that to them. My understanding is that
JP and JH have talked and that things are progressing to the point they would possibly be talking about compensation, etc. Again, this is from sources and not
JP himself so it's always up for debate.