Return fuel line should be rubber hose with metal braid over it.
Reason it starts OK within an hour or two is because there would still be fuel in the fuel rail, lower pressure but fuel none the less, as soon as engine RPMs get to 400 fuel pump comes on full time so you wouldn't notice the lower pressure too much.
I am just assuming from your description, after turning key on and off 3 or 4 times, and still getting no start or stumbling start that problem is air in the fuel rail coming in from fuel leaking out and over time air being pulled in to replace the leaking fuel.
Just a guess.
Based on fuel pressure dropping over first 30 minutes.
It could also be same situation but in the gas tank, leaking hose/line allows fuel pressure to go down then gravity leats more fuel drain off and it is replaced with air, so now you have air in the lines, although I would expect a good start and then a stumbling as the air was forced from tank to engine.
There is another test you can do, for leaky injector.
Shut warmed up engine off
turn on the key
press gas pedal to the floor and hold it down all the way
Key ON, engine OFF, gas pedal to the floor = Clear Flooded Engine routine in the computer
Computer will shut off Fuel injectors but leave spark on.
Leaving gas pedal all the way down try and start engine
it should NOT start, it should not even fire since there is no fuel coming in
If it does fire then fuel is getting in.
If it does fire, next test would be to let the engine sit for 10 minutes.
Unplug the coil pack, so no spark
Do same test as above, gas pedal to the floor, crank it a few times
Now pull spark plugs
Wet spark plug has the leaking injector