Don't attempt to connect the high-side connection with the compressor running. The pressure can be very high and you won't be able to connect the fitting without a whole lot of force. I connect both fittings before starting the car.

You should see the same moderate pressure on both ports when the system stabilizes after shutting down the compressor. As long as there is any liquid refrigerant in your system, this pressure is entirely dependent upon refrigerant temperature and not at all affected by the refrigerant quantity.

Certainly check for power at the clutch, and if you have pressure in the system you certainly can power it with a 12V line for long enough to check the pressures. If there is low refrigerant in the system, you will starve the compressor of oil so don't run it very long if the high-side is too far below specs while running.

Do you know if there is a compressor speed sensor on your car? These can be problematic from what I hear. I don't have one on my '96.