Replaced the oil pressure sensor again. Back to showing correctly in the dash. Last replacement was with a genuine AC Delco part, and so was this one. Hopefully this one lasts longer that a few months! At least it was within warranty.
ETA: regarding my replacement cluster, and fuel level, I decided to just leave it as-is. The over-full indication levels itself after about an eighth of a tank, then seems pretty accurate. Digital Dash Solutions did offer to swap it if I wanted to but I decided to keep this one, since it evens out after a bit.