Unfortunately, I can't give you vehicle specific data, but can you blank off the fuel rail to the injectors? (sealed injector pipes or proper union blanks).
If you're having to prime the fuel system, that says the system is not capable of creating the fuel supply required. By blanking off the injectors you can see if the system is capable of building the fuel pressure or not. If it can, the injectors are leaking the fuel out of the tips, (not the return). If it's still incapable of building pressure you know it's a fuel supply, regulator or pump fault.
At least then you can focus your diagnostic efforts.

Tony