That is an excellent price on a new compressor.

X2 what GM said. I have used Nylog on the o rings before reass'y. But I have had a couple not work and used a replacement.

Piercing valves leak. A crimped Tee in high side sounds much better.

Put a shower cap around the front of the compressor after verifying there I refrigerant in the system. Let it sit overnight and place a sniffer thru a small hole. se if the sniffer activates. If it doe, could be the shaft seal.

Could be blend door actuator, heater reheat problem, compressor control valve. Now that you have the correct refrigeration level, time to troubleshoot.

24 on vacuum is not enough depending upon where you are located. what is your elevation? Better recheck your equipment.

question 3... Some of the POA's were adjustable, some were not. question 1 I would keep it R12. there is enough R12 available. Corvettes were a poor candidate for R134a conversions. I still have some small cans of R12 and some with dye only...no sealer. question 2 VIR's for Corvettes usually needed ...

Oil can be added with an oil injection tool. If the refrigerant level is correct, you'd connect from high side to low side. Then you'd use high side to push oil in low side. This does take experience to do it this way as you do not want to slug the system.

This is the time of the year that there is not much activity here. I do check once a day or so. It will get busier once the weather gets warmer.

I only used to use copper sealing washers on flares that had a sealing problem.