Originally Posted by 97 SL320

He needs to pull trans trouble codes and clear them / drive until a fault then pull codes again before going any farther, don't guess. Also have a look at the ABS system as wheel speed info comes from that system.

Idling in gear for 1/2 Hr won't cause any damage as long as the parking brake holds.

The initial shift might have been the car going into limp ( this trans defaults to 2nd gear ). If it actually made it into a higher gear, the trans will stay in that gear until the engine is shut off. ( This is to prevent the trans from hitting limp / 2nd if there is an electrical issue while at speed. )

It is common for the case connector bushing to allow fluid to wick up the wiring to the trans computer, this can cause issues due to the fluid being slightly conductive. ( Graphite / metal dust in the fluid ) Cleaning the trans computer is possible. Mounting the trans computer connector down helps keep fluid out as it will continue to wick even after the case connector is sealed up.

All of that being said, diagnose and don't change parts at random.