Here's what I would try at this point.....

I would want to try and determine if the disc/input shaft is turning, when it's hot and the clutch is depressed.

Go out, drive it around, get it hot to where the problem is occurring.
When you get it back home like that, immediately jack it up and get it on some jack stands or lift(or if you have a friend with a lift, etc).
With the inspection cover removed and the motor running, brakes set, you should be able to see if when you push the clutch in, does the disc stop turning or not?

If when trying to push the lever into gear the disc doesn't stop turning........ Either there is too much drag on the input shaft in the bushing....... Or the brass ring just doesn't have enough friction to stop the gears from turning.