As a suggestion before putting the motor under the spanner.
Start it up, get the oil circulating, stop and let things settle. Sump plate off, let it drain down to ensure the crankcase is truly empty, sump plate back on. Disconnect Rocker feed and direct to the oil tank filler. Start up, and observe the oil return from rocker feed and the normal return flow. Ok, the rockers will be dry for a few moments of the test, but this would prove the scavenge works and the problem is oversupply to the crankcase rather than poor scavenge. Oiltank level should settle slightly, then remain constant.
The ball valve for the anti wet sumping is sealing the oilway to the timing bush when the engine is not running. So sure, no wet sumping on standing. But behind the ball is the spring and its blanking plug so when oil pressure moves the ball, this pressure is also acting on the spring plug. So a leak here feeds directly into the crankcase, and will only be apparent with engine running, and with oil pressure in the gallery. Something else to check, but awkward to do in practice, oil pump off, pump some oil into the gallery from a pressure type oil can and looking for rundown is one way.
Swarfy.