Found the problem, now looking for the culprit. [Update]

The fuel pump is not priming. Not with 3 or 300 on-off accessory clicks in the ignition. Nor is it priming with any of my keys. You can’t hear the pump either. I even took apart the backseat again so as not to assume the sound was muffled.

Does the fuel pump work? Great question: here’s what happened when I cranked it with the pressure gauge spliced in:

Can confirm: fuel pump pumps gasoline.

I’ve tested and swapped (I have many spares and multimeter) the fuel pump relay, fuel pump fuse, and EFI fuse: all good.

My brother-in-law told me I need to research the components of the prime circuit.

Oh goody.

[Update] The fuel pump fuse slot was occupied by a 10A when it’s supposed to be a 15A. Various relays were rotated and swapped. Inexplicably, the car has started four times over three hours on the first attempt, without fuel primed into the rail on the first startup (my pressure test was capped, not connected to the fuel rail). I have been using my backup key and tumbling two or three time prior to cranking. For context, the car had not cranked on a cold first attempt in nearly a week. I am going to attempt once more tonight when cold and see if it starts up again. Tomorrow, I will use the same key but without tumbling the ignition multiple times before cranking and see what happens.