If engine mounts were the cause of your rough idle in gear, you would know the difference immediately with new mounts. I just changed the motor mounts on my 1991 300CE and it is smooth as butter now. The tach reading is the only clue that the engine is on.

I am just finishing up engine mounts on my 2000 C280 to cure the rough idle in gear. The typical diagnostic is with the engine is running, raise the engine a little bit with a piece of wood under the oil pan to see if the vibration goes away.