How about taking a stab at the oil check valve issue? Personally I was thinking doing something like this guys setup but with an inline check valve to replace the function of the original.

https://www.dieselpumpuk.com/shop/manifolds/oil-return-line-kit-holset/

Not sure how he's connecting to the pan, Original is basically press fit, maybe Threading it

https://www.peachparts.com/Wikka/OM617OilSepCheckValve