I think you're really overheating. 100 is too high for normal operation...you should be seeing 80-85 under most operating conditions. My stab at this is that you have some local boiling due to heat soak when the coolant stops moving. You may have a plugged radiator or bad thermostat.

Also, how high are you filling the tank? It shouldn't be filled to the brim, as you need the air pocket for expansion. Anything extra will puke out, and the system should arrive at equilibrium at some level below the top.

Also, there's no need to pour water into the upper hose, as the Mercedes systems are designed to self bleed...if that isn't happening, it's a sure sign of a clogged system.