Should I let potential employers know I have a job offer?
The argument for plan 2 is that if you have an offer from employer A with a particular deadline but you would prefer to take a position with employer B, then if you don't say something, employer B could take too long to decide what to do and you'd be forced to take offer A before employer B gets around to making you an offer.  

I've been on the employer side of this (as both employer A and employer B.)  Keep in mind that once a search committee has completed interviews it has to meet to decide on who should be offered the position and then this has to be reviewed by higher level administrators (deans or vice presidents) and that the process can take time (days or weeks.)  It may be possible to expedite this process if there's a particular deadline, or it may not.    

I think that if you've decided that you'll take offer A by a particular deadline if you have no other offers (and that you wouldn't break your contract with employer A after accepting the offer), then you should tell the other potential employers about the offer and the deadline (I wouldn't mention employer A's name or the specific amount of the offer.) It's not likely to hurt your chances with these other employers, but it could help motivate them to get an offer out to you in a timely fashion.

I have received this kind of information from job candidates in the past.  Unfortunately, I've always had to say that (because we had other candidates scheduled for on campus interviews) that we couldn't make an offer before the deadline.