At this point, I would probably replace the main battery cables, both positive & negative (at almost 10 years old you can’t really lose on that) and then find every grounding point under the hood, pull them all, clean them up and reattach. Then I’d see where things stand. I say that just on the idea that if I had codes that seemed to indicate voltage problems and were moving targets then I’d first suspect a bad ground.
I’d still check the fuel pressure too though.