I think the end conclusion is regardless of oil youve got an terminal problem which will be resolved by
A.engine rebuild
B.second hand engine
C.sell the car for spares/repairs and buy another
D.set it on fire and claim on insurance lol
This. Plus E. do the 1000km the mechanic suggested and see how it goes (which costs nothing). Failing that A or B because if it's failed a compression test on multiple cylinders, the piston rings have probably had it. But it does have a fair few miles on it. Not sure how much you'd get back from D LOL.