My car does the same thing, in that it falls to a lower rpm before immediately raising up. Although in my case I believe it only does that while you're moving, not when it's actually at idle. (that's with the clutch in or in Neutral of course)

If you feel it's getting too low ( I'm not sure it's as close to stalling as you think it is) it could be an issue with the IACV (maybe needing cleaned) or maybe a ThrottleBody cleaning is in order.

Otherwise, as far as the A/C pressure switch - if the revs go up a bit when you turn on the A/C (right as the compressor is engaged) then I don't think that's the issue. The rpms may only stay the same when you turn it on, it's if the rpms drop and the engine becomes noticeably rougher that would indicate the ECU isn't getting the signal that the A/C is on or, again, could just be the IACV is clogged and needs cleaned.