The HST and PTO switches are 4-pole switches with 2 individual circuits. One circuit is for starting, and the other is for OPS. One circuit can fail, and the other can still work, so the tractor could be started but not driven if only the OPS circuit failed.
Just this evening, I diagnosed a BX23s (different tractor, same switches) that will not crank using the key switch. I started it by jumping the solenoid at the starter to get it running and I was able to drive it into the shop.
The point is, do not dismiss a failed HST switch.