If the compressor is not running, both service ports should show the same pressure. If there is any liquid in the system, this pressure is dependent on the temperature of the refrigerant only, and not on the level at all.

Oil on the compressor can really only hurt the clutch. The front seal isn't designed for petroleum oil, but you probably can't get enough motor oil on it to cause any trouble. I'm not sure, but it seems possible that a lot of oil on the clutch could keep the clutch from turning the compressor shaft, even though the magnetic coil is energized. I doubt that this is the case, and more strongly suspect that you have no current to the coil. Check for voltage there.

The only way to inspect the insides of the AC system is to flush it and look at what comes out. This is a huge procedure, but is certainly not impossible, nor beyond the means of a DIYer.