I once had a mechanic I worked with forgot to remove a rag from the intake port, another that had one wrapped one around the oil pickup when doing a in frame rebuild. I doubt this is the problem either, sounds like it is electrical.
Still comes down to pulling the plugs. is it possible that the wrong starter is installed. I doubt it, but that could be the thunk you hear.
I understand it rolled over before the mechanic worked on it? The idea of low voltage is a good one, when starters go out they sometimes pull more and more amperage, which is hard on the relays and connections.
Put voltmeter on the positive terminal of the starter and see what the voltage is when cranking. Would not hurt to do this on several connections as well the ground connections when under load as checking for voltage at the sensors.
To check loaded ground, put the positive lead on starter and the negative lead on the negative on the negative battery terminal. then hit the starter, if you get significant voltage it is a bad ground try the block to ground also.
I have replaced all the relays in my high milage pickup for intermittently shutting down, not recommending the same attack but I did test the relays and found the ignition one bad. Still ran the fuel pump, but died while driving.