Your Mom is on the right track. You need to find a Pit Bull that has a law degree. These guys (repair shop) will blow off a letter from the BBB, but as a matter of course, a good idea. You should also notify the State Board of Insurance. Your insurance company will not want them on their case, so that should get their attention.
Stay in the face of the MB regional rep, don't let him blow you off!
Most of all, you need an aggressive attorney who handles these kind of cases. You now have a car that is worth significantly less than before the accident, even if it was fixed properly! You have significant damages(financial), but don't try and "cheap out" on this by "doing it yourself".
They are counting on you getting frustrated and going away. If your case is as you say it is, a reputable lawyer who's any good at this, will take it on a contingency. That means he will take a pretty good chunk of what you get. But, again, if he's any good, he'll get you a bunch more than you will get on your own (so far zero).
I to say I told you so, but I suggested the same thing when you posted this last summer.
91 300E 120K
90 300SE 275K (sold)
92 BMW 525iM 120K
90 BMW 525iA 175K
85 300D 175K (sold)
84 300SD 245K (sold)