If I understood you correctly, there are no known freon leaks, it's just that the compressor is locked in most or all the time. It sounds like you already know that this is a control problem, not a refrigerant problem.
If there is no leak in the system, I wouldn't pay so much as fifteen cents to change it to R134. It sounds like it might be a slow day at your MB dealers service department. They're trying to produce some revenue.
It sounds like you and Benzmac determined the problem to be the MAS. It also sounds like this independent A/C shop which is highly reccomended is the way to go.
There are some really good Benz service departments around, but there are also some that are thinking more of producing revenue than they are thinking about SERVICING their valued customers.
I was in Calgary about three weeks ago and about a foot of snow blew in when I got there. It's hard to imagine worrying about the air conditioner. Of course, in you're case it's like you're trying to turn it off! Just teasing.
Good luck with it,
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in