Most MB fuel pump relays of that era use the starter signal to get the relay energized below 300rpms. I know of no inertia switches on MBs.
Have you tried bypassing the relay to see if the circuit is good from the relay to the pump. Jump from 30 to 87 terminals usually the two oposing middle terminals of the relay (verify).
Bosch Master, ASE Master, L1
33 years MB technician