Confusing why when you "short it" the switch works...
I recently had a strange thing like this happen on my truck.
It wouldn't start one day and it was obvious that the fuel pumps,(it has 2 fuel tanks and 2 fuel pumps) weren't working.
Like you I went through everything. It all checked out good. I never thought about the switch that changes the tanks. (Really just turns the pumps on and off) sure enough, the only thing that I was overlooking, was the problem... Bad switch.