A warped clutch disc could do exactly what is described. How do you warp a clutch disc?

By letting the back end of the transmission hang down while main shaft is in clutch plate but not yet in pilot bearing is one way. This can warp the disc enough to still have drag even with clutch pedal to the floor.

Question is: Did this all start after overhauled transmission was installed, or was it overhauled because of this problem?

In other words, when did this problem first begin!

Again, what happens when shifting from neutral to reverse? If grinding it is not an oil or synchro problem. It is most like a clutch problem, or remotely a mal-adjusted linkage issue.