I'm in SoCal with a '91 LS 3.0l and was going crazy chasing down an intermittent A/C issue for quite a while, so I feel your pain. I believe that I ended up using the '89 wiring diagram.
With regards to your questions:

Compressor: You'll know if the compressor engages because you'll see the clutch on the front of the pulley engage and start to spin with the belt. If it does engage, you should also check the pressure level of the refrigerant to make sure that it is full/full enough to keep the "low pressure switch" (mentioned below) happy.
Condenser fan: Does it spin freely when you try to move it by hand? If not, it is probably frozen/burnt out and needs to be replaced with a new pusher fan. I believe that they are 10" fans. Generic ones can be found online rather cheaply. If this fan isn't working, you'll likely not get much in the way of cold air out of your A/C.

It sounds like you've done most of the more obvious checks, so I'll just toss out a couple of the less common ones here that I went through.

Thermal switch. It's a weird relay inside of the dash assembly on the passenger side, behind the glove box. This thing basically has a temp probe in the Evap coil side and switches the compressor on and off based on the evap coil side temp. This ended up being the source of the issue on my Monty, so I just bypassed it with a blade-type fuse. I run the risk of icing up the coil, but I'm good with that slight possibility. This is also the area where two of the other relays involved in the A/C system are mounted (shown in one of the attached images).
Low pressure switch. This is on the high-pressure side of the system and is screwed into one of the hard lines behind the grill, on the passenger side. It can be tested with a simple continuity test.
Engine coolant temperature switch. On my 3.0l it's the passenger side sensor closest to the firewall.

Good luck!

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MonteroAC_ThermalSwitch.jpg (53.84 KB, 1 downloads)
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'91 Montero LS, LWB 3.0l V6 Auto. Rebuilt block/heads, E-Fan conv.
Happy to share my experience, but follow my example or suggestion/recommendations at your own risk.
Don't blame me if you damage your stuff. Use common sense and make your own decisions!