I had rebuilt an '87 740Ti A/C system from the ground up and also worked on a '91 740Ti I once had. Cursed the system as it is!
I am happy to have the totally manual '89 240 A/C system that has the simple manual thermostat and clutch circuit compared to the 740. Take heed from the advice below, and make the mods which avoid the issues you ran into.
One observation I will make on your use of GMC A/C relay is that it may be the sole reason why you have the intermittent problem with clutch and fan behavior, where the relay is not rated for the current the complete circuit is consuming, thus dropping the voltage to both and disabling one or the other, depending on which is on first. If you really wnated to fix that, put another control relay in the fan circuit (off the battery) that trips off of the A/C relay and it won't overburden the A/C control circuit.

The delay for clutch start is built into the system at the MCC, it's designed to keep the engine as unloaded as possible during start up. Why it's doing that when the fan is disconnected is another story. Most of the volvo 740 condenser fans I've seen recently have completely rotten fan connections. The wires as they go into the fan housing seem to have lost all the insulation. If you have a short there that could be sinking away all your current. Considering how wimpy those original 740 condenser fans are and the cost of a new one I would replace it with an after market hayden condenser fan. A 14” is 60 bucks, will fit and move much more air than the original. If you use the relay wiring kit available you can just tie back the volvo fan circuit and and use the clutch power as the signal for the fan relay. Source the fan current from the battery. This gives an a/c on = fan on configuration. If you want I can post an image of the bracket I made to mount this fan using the original fan mount holes.

For the 740 CCOT w/134a system you should see 40 -> 45 low side 290 -> 330 high side on a 90 -> 100 degree day with the condenser fan working, engine at 1200 rpm, the center vent temp should be 53 -> 65. If you are getting colder, good for you. If the fan is not working or the condenser is dirty your high side will be above 300 . Either way if the original r12 prossostat cuts out at 300 you are not going to get much out of the A/C system on a hot day using 134. The new pressostat for 134 I bought must be at 350 because I'd didn't have any clutch cycling and the gage was a little above 320 when I charged the last 740.

That pressostat on the high side, below the orifice valve, is available from NAPA for about 20 bucks, under the make and model. All of these pressure switches are also made by a company called santech (not scantech) and your local AC re builder can order the exact one you need. Also if you have switched to one of the newer zexel compressors w/o connections on the compressor you will need to tee off a high side 134a connection from that same high side pressostat connection if you want to have a hope of charging the system correctly. Note there is also another pressostat on the accumulator, that is the low pressure switch.

first things first; did you check to see if your condenser coil is dirty/blocking air-flow?

Turns out that my car has manual climate control, MCC, so there apparently is
no thermostatic control of heat nor AC. I hope to eventually substitute the automatic climate control, ACC module - would appreciate comments regarding the
feasibility of this.

Turns out the high pressure switch is used to activate the AC fan - don't know details - this function is mediated by the MCC module - but I think the switch should be open at low pressure. Too bad I didn't keep the box my "replacement switch" came in so I could return it.

Now plan to manually switch 12V to suction side pressure switch to activate AC - at least until I can swap out the MCC module for ACC.