First - I hate aftermarket sensors and actuators. Often times they just make you think that it can't be them (because they are new), and often times they fail quickly, or are even bad out of the box!
I like the IPR fitting as the first step for an air test. With it, you do not have to command the IPR valve closed. Many places have been sold out of that fitting, so I have gone to the one sold by SWAG Diesel:
SWAG PERFORMANCE FORD 6.0L POWERSTROKE DIESEL IPR AIR TEST TOOL FITTING
You need to look at the ICP pressure, ICP sensor voltage output, the desired ICP pressure, and the IPR % duty cycle (commanded value), to decide if you have an issue in the high pressure system. I would do that before doing an air test, but nothing wrong with doing an air test - just the effort involved.
The parts stores do not have the best diagnostic software tools. They usually do not do a complete
scan. Too often they miss things.
I am not the best one to recommend wire size. @TooManyToys
would get into those details. You could search and find a TON of information on the subject (in this forum AND on the internet).
I have a 230A alternator, so my choices are not what you need stock ...... and the price of copper keeps going up. My wiring to the batteries and between the batteries is 2/0. There are also ground updates that help keep things happy (mainly processors). Also, I have two additional batteries under the passenger frame rail for power out in the "sticks", and for emergency power, so my system isn't one to compare to.
IMO, you are better off with the charge wire to the passenger battery (shorter run), and then making sure the batteries have the proper size wire between them, and to their ground points!
2AWG is probably good enough for a 90A alternator and a short run to the passenger battery. Also, the starter is powered off of the passenger battery, so you need proper power there! Then you need something adequate for proper charging of the driver's side battery (and to ensure glow plug operation and processor power).
My compliments to you on resolving your fuel pump relay. That relay is the ONLY thing that has stranded me, but I did have most of the parts purchased for the installation of a separate (external) relay, so the issue wasn't a big one for me. Unfortunately it can be a BIG issue for most owners, especially if it happens on a long trip.