it appears that in all of our line purging, we missed the one line that goes from the fuel filter to the far side of the injector pump (between the engine and injector pump). I'm not sure if that's THE input to the IP or what comes from the return lines from the injectors, but after I loosened that banjo, did some pumping, then retighted it, it took several minutes and finally started up!

After spending more than a few hours on this over the past couple of days, I was about to tow it to the shop - so I owe you all!

Jason in Austin (thanks David for the tip about the tank screen - may have to use that next time!)