1. That EFI service port is like a tire valve. Remove the cap and there are threads a pressure gauge is supposed to go onto. A high pressure gauge... able to read at least 50-PSI. If all you want to do is see if there's pressure, cycle the ignition to RUN a couple times, then press the valve core like you would on a tire. It should really spurt since the pump should deliver around 50.
2. Yes it will depressurize the fuel line. When I changed ours, I just went under with the release tool and let it leak. It'll leak with no pressure, since the filter is low and both ends of the fuel line will drain down. You'll find the filter is in some kine of clamp or bracket. If I recall correctly, E450 filter just snaps in place. If not you'd need a tool to release the bracket.
Will try to find the thread I started on it.
