You said you replaced the master cylinder, but it might be defective internally. The bore diameter could be slightly out of tolerance causing fluid to leak by the seal or there is surface corrosion. Remember standard brake fluid absorbs water over time which can create internal corrosion related issues throughout the hydraulic system.
I can see you are trying to do an in-depth engineering analysis as a way of solving your problem. On a personal level after reading about what you have tried to diagnose on your own, I would seek the services of an experienced MB mechanic. Many times, these people are not working for MB dealerships but instead work for small independent garages that only work on MB, BMW, etc. type vehicles. My experiences using independent types of garages has been very positive. They analyze the problem, call before doing any work, explain what the problem is and provide a price. I like to work on my vehicles but there are times when the services of the experienced is the better way to go.
Jack