"But anyway: when I launched the first ones at a jump point (the minelayer was parked on top of it), the mines immediately launched their missiles. Which didn't move at all and ran out of fuel. WTF?" -if mines are launched at their target they immediately dump their payload for some reason, launch them at a waypoint(you'll want to do this to space out the mines properly in any event).
also 3 stages don't work, I think for the same reason.