A/C only blows warm air although the engine changes idle speed when the a/c is turned on so I'm assuming the compressor is trying to work.

After the a/c ran for 10 min both lines leading from the compressor were warm.

Next step I suppose would be to check a/c system pressurization. My experience with a/c systems losing pressure in the past resulted in a slow loss of cool air where it gets less and less cool until the air finally isn't cool at all...which coincides with the 'freon' slowly leaking out. Since this happened all at once should I assume there's a major leak somewhere and should do some kind of system pressurization test before I dump more freon in? Or should I be looking somewhere else in addition to checking freon?

You are on the right track.

With the intention of using the gauge I got a can of refrigerant from the local parts store and hooked it up (separate from the bottle). Although before I assumed I had a leak, turns out my system appears to be OVER pressurized instead of under. Not sure how that happens at all. Also, my serpentine belt just started to squeak. Never has before and seems tight, so dunno maybe the two problems are related.

With this new info in mind is there any advice on where I should look now for the problem?

If the belt has started to seriously squeal, your A/C compressor clutch may have given up the ghost. Turn on the A/C and look at the compressor to see if the belt is turning the compressor. If not, the clutch is probably toast.

Good news looks like I may have narrowed down the problem. Turned the a/c on and observed the inner wheel of the compressor to be totally stationary for the 15 minutes I watched it. And the new squeak I'm hearing seems to be coming directly from the compressor.

Is this enough info to give a definite diagnosis of a bad compressor clutch, or could it still possibly be something else?

bump