OK,I am not questioning the diagnosis of your mechanic, but, a word of advice here,
Are you 100% sure the IP is plugged, or is the control rack stuck in the "fuel off" position? The Mack does tend to do this if left sitting for extended periods.
Did you check that the supply pump on the side of the IP had not dried out and was unable to draw fuel from the tank?
Did the emergency shutoff valve get bumped accidently?, I cannot remember if that truck still had it installed.
Once again, not making any judgements here, just offering options.
Fuel shutoff was the second thing I checked (check that there is fuel first), cover the easiest and cheapest things first. Moved freely and later tried it in multiple locations during starting attempts.
Removed the fuel line from the transfer pump to back flush it, then reconnect and prime it from the tank, good there.
Worked up the fuel system to the filter and whoa, there's the problem. Nasty gunk in the filter housing and some of it was preventing the filter from seating correctly, allowing gunk to bypass the filter and go directly into the injection pump/injectors. About a handfull of this I flushed out of the pump. Mud, rust, and some other hard particles are in there.
Flushed the pump out until clean fluid was coming out of its supply/return ports then reconnected them to a cleaned (up to the pump) fuel system. Reprimed, still won't start. Remove the readily accessible hard lines from the top of the pump and crank it over, no fuel coming out. Do some more cleaning and now 2,3,5,6 lines are spraying as they should. Nothing from 1 still, and 4 is not readily accessible (just takes longer to reach). tried restart again at that point with no success.
Which leaves diagnoses. That gunk is still plugging up part of the injection pump, to at least #1. And highly likely some of it made it's way into the injectors and causing problems there since the injectors have the tightest tolerances.
Valid point and I really, really wish it had been something as simple/normal as overlooking the fuel shutoff.