You can have 12volts at the compressor and a defective connection. Use a test light at this connection so there is a load. Most voltmeters do not create a load.

If my failing memory is correct, the POA has an oil bleed line on it. There is a shrader valve in it that is SPECIAL. Do not use a normal replacement.

Early Corvettes were a hard vehicle to keep cool. If I were you, I would keep it original and keep it R12.

So you just vented the extra refrigerant? You're supposed to recover it; not let it get into the atmosphere. Now you do not know how much is in the system.

Check clutch gap. I did not read your entire post.

Recover the charge and weigh the total correct charge back into the system. You may have a marginal TXV at the rear evap. Was it changed?

The rpm at idle is not high enough. Run the engine at 1500 rpm. The high side appears high; it may need a new fan clutch. Run the engine up, mist the condenser with water. If the high side drops, you have air flow problems across condenser. The system may also be low on refrigerant indicating multip...

You might try an internet search on the blend door on the Sierra. Common problem. With the heater by-passed for the summer, your defroster will not work.

reheat means the air flow is going thru the heater core. Easiest thing to do is eliminate the heater core. ( Yes, by-pass it)

Have you blocked off the heater core to see if you have a reheat problem? Are the blend doors functioning properly? Leave the refrigeration system alone until you have checked for a reheat from the heater core.