Rich, I agree that there is a small amount of mixing over time. With thermal expansion while the tractor is in use, the fluid is expected to climb up the riser pretty much up to the filler cap. That results in there being maybe an inch and a half of about one inch diameter riser in which the two lots of fluid mix. However there should not be any flow through that riser, so the amount of mixing should be very limited.
Bigger, better hydro transmissions have two separate fillers, and perhaps two different lubricants. The K46 is not only made to be cheap, it is also made to be very small so it will fit small tractors. I don't actually hate the design of the transmission, the problem seems to be tractor manufacturers who put it in situations where it will be overheated and overloaded.
I'm really looking forward to seeing the pictures. Right now it looks like the first couple of chapters of a murder mystery.
