Come to think of it could very well be the delivery valve I guess for no line output. I suspect it is more the pump piston for that #5. I believe you can push it down to where the crank will drive it up again if it is sitting at the top of its bore. Thats if it"s not seized too bad. The pistons in the pump do not have connecting rods so only the residual fuel pressure or a weak spring retracts them back to start a new pump stroke I imagine. Push it back down if you can and spray some mild penatrating oil in there. Not the strong acid based stuff. I do not think this problem produced the large bang the owner reported.
Sitting from 2001 is a better possibility for the pump problem. Looking for the source of the large bang might become interesting soon. Your compression test sounds like a place to go even before working on the pump although you have pretty well proven it has a problem. Make sure the cam is turning with the engine as well. Hope the large bang was not terminal.