Trying to help a friend long distance with new 560 SEL. It was changed to R134 by owner; did not work when friend purchased. Friend has done this:

Replaced compressor with Denso.
Replaced all hoses.
Flushed entire system
Replaced expansion joint.
Replaced accumulator (dryer bottle)
Recharged with R134a. Apparently no leak.

Did not cool. At one time temp measured 50F while charging but later only 80F at 80F ambient temps. Had very high high-pressure side per gauges and compressor continually cycled on/off. So he decided to go back to R12.

Replaced Schrader valves.
Flushed system.
Replaced accumulator again.
Drained compressor and put in 4 oz mineral.
Put 3 oz mineral oil in low pressure hose
Charged with 42 oz R12.
Result - not cooling (80F again) and low side pressure measured in spec (think he said 50 lbs) but high side too high at 300. He said high pressure measured too high after only one 14oz can of R12 installed (not sure what)

What to do?
Could condensor be clogged and cause excessive high pressure
Could fan clutch be bad and not putting through enough air
Switch out climate control - maybe original bad
Jump the compressor - how to do - if the compressor is not coming on though, how could there be 300 lbs per gauges

As a diesel guy and with little knowledge of AC, I am over my head.

Any comment welcome.

Thx.