TBH I think you are clutching at straws with an oil leak between the transmission and engine being the root cause of the problem. It may be that seal is bad but I think your symptoms point to something in the engine being the problem.
Would have to agree on that, first off if that was the case a quick check of engine oil level would let you know if it was way over full. I know some engines are more sensitive regarding oil level.
It just seems like all that would be needed to prove or disprove that as a cause would be to check engine oil level and adjust to proper level and see if problem goes away.
Just had a thought, are you sure the dipstick is the correct one for the engine? If the wrong one was installed maybe the engine is just plain over full?
One more thing I don't recall reading any history about this machine or problem. Did the machine run for several years and then this problem started? Or did you just come into possession of it and it has done it from the first day you saw machine?