I still agree with your first thought that a "leak down" problem should not have much to do with a low oil pressure problem.

It's got to be either in one of two areas. 1) you have an actual "oil pressure" problem. And you have already checked and eliminated the oil pump so maybe you have a scavange pump oil problem and the pump is sucking air, which is then causing the erratic oil pressure readings.

Or 2) you only have a problem with the "indicated oil pressure". You would need to hook-up a quality test gauge "not mounted to the vibrating vehicle" and see if you get the same original pressure results. Whenever I have found the cause of one of these difficult to diagnose problems, it always turned out to be something simple, or dumb. Hang in there.

Just some ideas