Toward the end of my visit, I came up with an overall theory to explain multiple issues here. I think that the controller stuck on (some time after the one new glow plug was installed) and burned out the glow plugs. It was disconnected and a manual control was installed with very poor wiring, but the glow plugs weren't changed out. I think that the truck was driven for a while like that. It would have taken more cranking on the starter to get it running like this which would have started to wear it out and cause it to turn the engine over slower. As all of this came together, the PO finally got tired of it getting harder to start or maybe it got cooler and wouldn't start as easily. Either way, I'm guessing that the PO couldn't get it to start and just gave up on it. The excuse that he gave to Derek was basically "I'm too old to try to figure it out". As we all know, that's a crock and a cop out. It's also used by people to cover up their bad choices that they used to try to rig a truck together to make it keep going.
If my theory comes out even half true, I'll be very happy since I was tired enough that I was maybe operating at 50% while I was there. Overall, other than some stupid, lazy things that were done by the PO, I think that this may be a pretty decent truck assuming that the engine is still good. The body is pretty straight with little damage.