Certainly smells like fuel pump(s).
Try this low-tech diagnostic method:
When it is not starting, try placing a hand on each pump while someone turns the key to the on position. You can feel if they are operating for a few seconds.
I say use hands because my digital voltmeter is too slow to register a voltage within the short interval they operate.
I suppose you could also use a test light...
If one fails to operate, give it a firm whack with something, and see if it kicks in. If it does, you know the culprit.
1986 300E 5-Speed 240k mi.