[Schurkey]

How do you know the hard starting ten months later is the same problem as hard starting previously?

Connect a fuel pressure gauge, diagnose pump problem instead of assuming pump problem. If it's the pump, I'd think the shop owes you some warranty work.

 

[Schurkey]

Where does the '05 hide the fuel pressure regulator?

Dropping pressure could be because of back-flow through the check-valve, or forward-flow through the regulator.

 

[Schurkey]

There is no externally accessible FPR on the '05s

Yeah, but is it integrated into the pump assembly, or is it serviced separately?

The OP is blaming the check-valve, but this pressure loss could just as easily be the regulator. If the regulator is part of a "Pump Module", it doesn't matter--the whole works gets replaced; and perhaps under warranty. But if the regulator is separate, that might explain repeat fuel pump replacements--the pumps could have been fine, the regulator was bad.

I've never had an '05 "pump module" apart. The only fuel pump I've ever dicked with on a GMT360/370 is my '03--and the regulator is up by the engine.

For that matter, there could be a leak in the fuel plumbing inside the tank. Years ago, there was either a rubber hose or a damper assembly that connected the pump to the steel tubing. That hose, or the damper could rupture and leak inside the tank. I expect the same could happen with the integrated "Pump Modules".