I'd bet on a bad ignition switch. #1, don't run it again with the starter engaging, busted teeth on a ring gear would be no fun.

A worn cam or stuck contacts inside the ignition switch would cause the starter to stick 'on'. I browsed your earlier posts and noticed the random stall. Since the stalling seems to be somewhat associated with the steering wheel that further implicates the ignition switch.

Since I made a post a while back about the ignition switch on my 560sl I had several requests for the removal procedure. It's on the net here It applies to the 560 but our cars use the same switch so most should apply.

If you decide to pull the switch... the connector pins are numbered. Pins 30 and P30 are power input, pin 50 goes to the starter. This circuit should be open in all positions except 'Start'

You could test the starter solenoid by disconnecting the cable between the solenoid and the starter motor itself then trigger the solenoid and listen for it to snap in and out. Depending on the access to the starter that may be difficult.