It sounds like your mechanic is on the case. Those are all the thing’s I’d have thought of. I probsablyu wouldn’t have changed them out until they were proved to be not working, but that’s a judgement call.
Pay att’n to what @longprime is saying, seem sto be speaking from experience. If it’s not that, I’d guess next to check the fuel pump relay, O2 sensors (if you have them on a diesel), or the fuel injector pump.