I'm just reading this now, but when you said it started fine if you opened it up, I though leaky injectors. If it was a bad check valve on the pump the throttle wouldn't help at all, it would just add more air to a too lean condition.
Despite what your mechanic said, a fuel system should always have pressure. It should change very little from when you turn it off, to when you start it a few days later. If it's dropping off then there is a leak somewhere. If your not smelling it, then it's likely at one of the ends, pump or injectors.
A easy test: If you crank it and it doesn't start, sniff the tale pipe. If it smells like unburned fuel then you are getting some fuel. This will let you know it's a leaky injector or an ignition problem.
If you prime the system and it starts off running crappy like one cylinder is missing this is also a sine of a leaky injector. The cylinder it's leaking in to has too much fuel to burn. This shouldn't last very long though, as it will pump it out in a few revolutions.
If it was my car (I'm not recommending you do this), I'd let it sit for a few hours after running. Maybe prime the fuel system a few time during this time. Then pull all the plugs and sniff test each hole. I bet one will smell much stronger.