I'm sure you have checked all of this already, but I thought I would post just in case. I had a case somewhat similar to yours, although I did not have the volume of coolant that you describe. I was losing some coolant, however, and immediately thought that it was the water pump as well. I took it to the dealership, and after the mechanic took one of those reverse mirrors, he fould a culprit hose that was ever so slightly cracked, and was causing this problem. It was WAY under the car, and you really had to strain to actually locate the leak.
Maybe if the car were hoisted and a similar inspection done, you might be able to find a hose that might be cracked or not sealed properly. Wouldn't that be great if that's all it was??
WOW! The water pump replacement cost seems to be much higher on the MB's than ordinary cars - is that an accurate statement? I've never had one go bad on my previous cars that I owned, but there must be a good amount of disassembly involved in order to replace it.
And of course, mystery things like this ALWAYS happen right before you're ready for a road trip...go figure!! Will say an extra prayer that things will get resolved and Sadie will get you safely to your destination, coolant and all!!