What the pump guy is telling you makes a lot of sense. That is valuable info from a guy who knows his stuff. For what it's worth, I also agree that from what you say It does sound like the injectors are stuck or clogged or something.....and not unlikely because with that few running hours over that many years you can expect that diesel fuel does get water and bacteria in it. So what you saw isn't surprising.
But dealing with the injectors is usually pretty quick and easy. How much are 4 new ones? Not much. If it came into my shop I'd probably clean out the fuel tank, replace the fuel filter, slap in 4 new injectors, and hope that there wasn't too much diesel fuel generated crud in the lines - or that at least I'd gotten the worst of it. I might even part credit the job if I looked at your injectors and they looked good enough to be rebuildable. Many are; don;t know about your type.
BTW, it's easy to spray pattern the injectors against a piece of cardboard as you take them out.... and get a rough idea of how they are performing. Most mechanics will do that out of curiousity & to help their diagnosis.
rScotty