This is a strange one.... that's a lot of stuff getting into your crankcase... on our non-yanmar the injection pump is completely independant of the crankcase/block so I'm having trouble visualizing how that could be the source.
The most direct route for fuel into the oil sump would seem to be via the combustion chamber, past the rings and down the cyl walls - but that would mean one cylinder wasn't firing and you'd know that in a 2 cylinder engine right away.
Failing all that the only other thing that comes to mind (that would pass that kind of volume) is the return line that should be going back to the fuel tank.. way out in left field, I know, but by now you must be looking out there...
