I don't know about the overheating, don't smell or feel anything unusual from the transmission. I spray the unit off after about every third mowing. The fan is fine.
Is there a way to check the fluid level? I understand it's non-serviceable. There are no leaks so if it is low, it came from the factory that way.
Brake assy is clean and functioning fine.
mower deck is clean
Can't say about the broken wheel key, but last night while trying to finish mowing I literally had to reach down and give an assist by pushing on the drive wheels to get moving and it didn't matter which one I rotated. As I said, it slows more and more the longer you mow.
I replaced the drive belt and there is no issue with the pulleys or belt.
I'm going to run it tonight as if I were mowing but without engaging the blades, may even take the deck off to see if it acts the same.