Not to doubt Steve's wisdom whatsoever (correct me if I'm off on any of these points Steve...) but I've encountered a few situations where ASR has made a oversteering slide MUCH worse if the slide gets going before ASR catches things. Could be reads the slippage, cuts throttle, applies brakes in an attempt to slow the spinning wheels but since the oversteer is already going this does the infamous "yank the handbrake" move by throwing polar intertia to the front (hence reducing traction at the rear). Physics, electronics, etc aside it tends to make the car "wag" for quite some time before regaining its senses. (far more than just a regular oversteer slide)
ASR seems to stop any chance of power induced oversteer 99.9% of the time but occasionally (or when/if) a tail-slide gets going (for whatever reason)....look out it can be more than a handful as it doesn't behave as you'd expect a normal car to. Forgiving is the last term that springs to mind.
As for why your tranny behaved the way it did, I'll leave that to the wisdom of Benzmac and Brotherton. My only question there is what was your throttle position activity for the few seconds/mins prior?
I'm glad you and your car are generally okay. Only thing is...would you of had time to hit the ASR defeat switch soon enough to effect the outcome? If you are planning on gasing it around it will most certainly help (autocross, drags, empty parking lot, etc) In normal driving (ASR "safety net" engaged) the action of ASR would be my primary concern. In my experience MB ASR'd vehicles don't behave as you would expect from a traditional rwd vehicle at the limit. Some time in an empty parking lot with some cones and/or a driving course could go a long way to getting used to how the car behaves at 9/10ths to 11/10ths. Before doing any of that I'd want to make sure everything in the car was running as it was supposed to.
Glad you are okay...Lee