Your description of the relay location is correct. I'm sure it is the fuel pump relay because I have pulled it out while the car was running to de-pressurize the fuel system. The car will run for about 1 second after you pull the relay.
I would pull my backseat out and check your part number but my 400e is on loan tonight. I'll have it back tomorrow AM and will cross check the relay part number tomorrow. I'm 99% sure you have the correct relay based upon your description.
Hope that is your problem.