OK I was wrong about the rail sensor needed to start.

It does need a crank sensor no matter what. If you say it's been deteriating over time, drop it and see if it's cracked. A cam sensor will make it hard to start and ungainly until it gets a reference signal from the crank sensor.

So! Check your crank sensor. You can check it with a piece of metal and an ohm meter. Far less money than rail pressure sensor. . . .