I have had the same thing happen on a 4650. Did all the thing you did, still no help. I even put in a K&M air seat in it place, but hated the ride it gave. While the air seat was install is when I discover the fix for my tractor, but did not realize it till later on. When the air seat was install, I had the return line going to the transmission cover plugged with a bolt to keep out the dirt. One day oil was spraying out from this return line, which was suppose to have zero pressure in it. Got a new bolt and this time clamped it bolt on to the line.

After I decided the air seat had to go, I was going to fix the hyd. seat with what ever I needed to get it to work. Put a used seat valve in, no help. checked out a few other things, no help.I was talking to a service guy about this and I had mentioned that the return line had pressure in it and sprayed out oil. That, he said was not to be the case on a return line. He had no clue as to why the return line would have pressure in it.

So I unhooked the return line from the hyd. valve under the seat and feed it into a plastic jug. Now this line has no back pressure and the seat works great. Takes a long time to fill the 1 gallon jug. The line just sticks in to the top off the jug so the oil does not get sucked back into the valve. When the oil in the jug gets to the return line in the jug, the seat does not work as advertised. Empty the jug and go to go again.