I have seen many of these, when they do what yours is doing it's almost always the pump. You can block off the return line and see if the pressure still drops, that would rule out the regulator leaking to the return line. I'm pretty sure there are no rubber hoses on that, so to block it off you'll need to get a quick connector of the right size and cap that and install it on the fuel rail. Or depending on what adapters you have for your gauge, you can install it on the return side of the fuel rail, that's what I do but my tester was expensive and has every adapter imaginable. I'm not too sure what all comes in the DIY price range sets. DO NOT pinch the hard plastic lines shut, you'll damage it. To check if an injector is leaking, cycle the key on and let the pump prime a few dozen times. If it starts and runs normally after that, and blocking the return line doesn't help, then it is 100% without a doubt the fuel pump.
You already checked for gas driping to the ground, right?
