Good to hear it was something simple.
I don't know how many electrical differences may or may not exist between my 1986 US market 740 and your 1991 European market 940, but on my car, the exact same phenomenon of intermittently blowing fuse for injection pump power was caused by a chafed wire to the A/C compressor clutch. The wires were riding right on one of the A/C pipes, and had rubbed through in a spot and were shorting to the pipe. That circuit shared the same fuse as the IP/GP system and would pop the fuse and stall the engine when the wires shorted. It was hard for me to find because it would only happen intermittently when the engine moved a certain way, AND could only happen when the A/C clutch was being energized. But once I realized that it was only happening when the heater was running in the Defrost setting, I had a clue.
You could start by checking that in case it's the same. Not sure what else is on that circuit in your car. Check all the wires around the injection pump too. All the emission control switches (on the throttle lever) and the altitude compensation solenoid in the bottom outer side of the injection pump are powered by the same main IP circuit as well. Same with the wiring to the EGR control solenoid, if your car has one. If any of those is disconnected and able to short to the IP case or engine, or if any has a chafed wire, then that will cause your issue. And finally, the glow plug control system also operates on this same circuit, so check the 4-wire harness going to the glow plug relay in the left hand side of the engine bay for any wires with bad/chafed insulation that could be shorting to each other or to ground somehow.
Agree on what you noted at the end. That is also something true that I have seen (and almost been tricked by a few times). If the solenoid is not opening, then fuel WILL still make it out to the injectors when you loosen the union nuts and crank the engine since a little bit of fuel does still make it past the solenoid plunger -- BUT it will only be a tiny amount, not nearly enough to start the engine. If the solenoid is working correctly and the pump is primed and turning, you should see big squirts of fuel out of the unions when cranking, not just a couple of drops.
When you see the big squirts, then assuming the GP's are working and everything is in correct time, the engine will start.