Sorry to hear about the problems. Sounds a little similar to my situation now:
https://xoutpost.com/bmw-sav-forums/...s-coolant.html
I'm at about the same mileage. My compression numbers were way higher (180-200 psi), so I'm currently tearing it down to replace the head gasket and anything else, unless it is beyond reasonable repair.
In my case, the expansion tank had been replaced about 30k miles ago, so it did not explode, rather it blew out a small enough hole in the upper radiator hose to release the pressure. After replacing both radiator hoses and a new radiator (old one was fine) pressure would build up in the cooling system (I believe from combustion gases, rather than overheating) and the pretty new radiator cap would release coolant at 2.0 bar.
I did a reverse pressure test that confirmed coolant getting into cylinders 2+3, but given the high compression, I'm optimistic enough to try to repair it.
A test for combustion gases in the coolant will be helpful in diagnosing, and is very easy to do. EDIT - ... if your cooling system is intact.
Regarding your thought to slap a replacement cooling system on there to dump it, you may find yourself in the same situation I was in - if the problem involves a leak from the cylinders to the cooling system, you'll get pressure and it will need to go somewhere, and there is no external fix to that one.
Anyway, hopefully my similar experience and tests might help in your case.