#5 above sounds very logical. If it's not that you might check to see if there's a loose fitting on the suction side, in other words, between the tank and the mechanical fuel pump. If there's a pin-hole or loose or cracked fitting anywhere along there, it would suck in air and airlock.
Another possibility is that there might be something wrong with the mechanical fuel pump. Engine runs okay when primed with the hand pump, but then stops when the mech. pump won't keep up.
I've never seen that problem, but one way to eliminate the possibility would be to start the engine, and then work the hand pump to keep pressure on the injec. pump while the engine is running. If the engine doesn't stall out as before, you'll know more than you did when you started.