Is there any chance that you have the filter I/O plumbed backwards? No filter should have that much pressure drop across it, and not 1 vs another. You mentioned what sounds like a thermostat/diverter in the oil system- even though it should bypass the cooler I would consider bypassing it too. Keep at it, try to get oil in every part of the system "manually", and if you can, fill the filter before installing it. I've had dry-sump systems that were a ***** to bleed, and my water system was hard to bleed on this car for no obvious reason. Maybe consider connecting a line to one of the ports on the filter and cranking to see flow and maybe burp air.