Hello Chris,
Saying it as it is:-
The patient has been diagnosed with a serious heart complaint, and as a result, more organs have been damaged to a life threatening extent, and you are fussing about relatively minor detail of slightly low compressions etc. The engine is almost certainly damaged to an undesirable extent and you now want to do fairly major dismantling to try to find out that extent.
I think it’s worth you just leaving this well alone except buying a new oil pump and sucking it and seeing, or decide not to throw anymore good money after bad -at this stage - when you should have taken notice some year (or more?) ago when the oil pressure warning was doing its best to let you know.
For goodness sake just fit a new oil pump, assume the worse and hope for the best!
Kind regards,
Gareth.