You car not shutting off is indicative of a vacuum problem. There is a vacuum switch connected to the ignition switch which is responsible for shutting off the engine. If this switch goes bad or is not connected, your car won't shut off.
I am really sorry for your predicament. I am surprised that you got this kind of treatment from an independent. My motto is, "If you burn me, you can only burn me once, once". I think that was from a movie. Anyway, I would take it to a dealer until you get some suggestions from users in your area. A decent mechanic would at least eat the labor costs from a mistake, but if they make no concessions, run for the hills.
When it comes to a Benz a dollar of maintenence is worth a $1,000 of repairs. Cutting corners only causes more headaches.
Then again, what do I know?
'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)