Did you find a solution yet?

These are just general guesses from obvious problems that could cause this
(without knowing the serial number i cannot say for sure because systems change over the years and so do configurations and components).

first i would make sure the steering switches in the control handle are not stuck, one of them might be.
as far as the motor not shutting off?
since you had to physically disconnect the battery check the line contactor for the motor, it may be welded shut.
what was the state of charge of the batteries? if they were low the arc'ing from the low voltage/high amp draw may have caused the contacts to stick together.