Try turning the compressor by hand with the engine off. It should turn rather freely with slight binding on each stroke.
The way you describe the problem the compressor has had it. Unfortunately this type failure is about the worse thing that can happen as the system pumps its burned up pieces throughout the system as it dies.
Aside from the compressor one must check the whole system for contamination. If it is black inside, you better read up on black death and replace at least the condenser also.
Bosch Master, ASE Master, L1
33 years MB technician