If the oil return pump is doing it's job and the oil collects in the crankcase after sitting then I can't thing of anything else it could be other than the check ball/seat or possibly (less likely) a failed oil pump gasket. I had a sumping issue with my pan a long time ago. I bought a couple of spare check balls and after cleaning the pump seat area I dropped the old ball in and gave it a couple of taps (not whacks) with a drift. Then I replaced the old ball with new. I never had sumping problems again after that.
Usually with an imperfect ball/seat seal it takes a while for that much oil to collect in the cases (sometimes a couple of weeks). If you get that much after after each time you run the motor then something is really keeping that ball unseated.