There is no way to remove the oil from the compressor without removing the compressor itself (as far as I know).
Years ago, my '86 300E was having sort of the same issues. The main difference was I had no R12 left (empty). I pulled a vacuum and dropped the hydrocarbon on it, and got cold AC for years after that (I guessed there was enough oil in the system).
The hydrocarbon molecules are bigger than R12 and R134a, hence it leaked very slowly, I only needed to add a can every summer (5 bucks). I still have like 8 cans left.
I am currently working on an E420, AC compressor is seized. I just got another one from a junkyard (Denso 10pa15c, the original on the car is a 10pa17c). I will be installing it this weekend.
I will add some ester oil to the compressor (it will do OK with the original pag already in the system), pull a vacuum and drop 16 ounces of "Enviro Safe".

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1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
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