Just got my first slant six

Those compression readings are waaay low. The reason I questioned the readings was that if the cylinders are worn, it would be highly unlikely that they would all wear that evenly as to drop from around 120 psi new to 75-80 psi on all 6 cylinders. Your 7 cranks sounds good, as well as the thread-in tester. But I really suspect something is effecting it in common.... timing chain slipped, exhaust pipe plugged, etc.

I suppose the choke may be effecting it; just open the throttle and at some point you can manually open the choke plate by hand and put something in there like a socket to keep it open.

If you dropped that much in oil weight, the pressure is going to get considerably lower.

The flaky grunge sounds not too uncommon. Again, was there any smell of gas in the oil that your drained out?