Holy Crap Brad!!! YOU have done your research, I've never seen anyone who has covered the basics as well as you have here
I'm kinda at a loss but, I'll tell ya what I'd want to check if it were me.
First, nothing gets you there faster
than the right scanner (and here I know you talk about your buddies scan tool BUT, most of them aren't really much good for our trucks). You need a professional scanner, most here use the AutoEnginuity for personal use, if you have an Android device the Toque App can be very useful for pretty cheap too.
You say this happened after it rained. I'd be thinking about the IDM because of where it's located (actually outside the truck in the fenderwell) it is a common failure point from water getting inside. You should have a SES light with a P1316 though. In any case, it's a pretty easy thing to swap with ANY other 7.3 IDM for a quick test without the real diagnostic tools to confirm if yours is working or not (you know anyone who you could try this with?) Also, look above the fusebox, at the GEM module, for evidence a windshield leak has been dripping into it.
Fuel pressure, I'd want to confirm you had pressure if you can get a mechanical gauge on the fuel system.
I guess that's where I'd start. See if you can get the right scanner connected to monitor the sensors and pull codes (again a regular OBD-II scanner will say "No Codes Found" when there Are codes there). See what it reports from the IPR & ICP (it's the ICP on the ft head, and you were right to try that
). If you can find an IDM to try I would do that (worth a shot...) and being sure you're making fuel pressure is another good thing to know to go from here...