Stop grinding... Just my 2 cents. But without a measured mathematical problem, you chasing ghosts.
Check the crank for errors. Check the block for errors.
Also, rear main seals have very little pressure on them. The oil that they stop from pushing through is from splash oil. It's not a pressure seal. It's mostly a barrier seal. You have a gap somewhere in the crank to seal lip.
Either the crank is out of tolerance or the block is off.
I know this is frustrating. I was there. Lots of advice. Most is urban legend. I even bought a rope seal at one point. Engine builder laughed. Refused to put an inferior product in the engine that would eventually fail. I was to the point of buying a new block.
My engine builder did build a aluminum billet crank plug to the exact specs of the crank. He used that to look at the issue.
