[Suicide Junkie]

A missile will disarm and vanish if its target is destroyed.
The plasma missile's damage depends only on how far the missile travels.
Think of it as using the plasma from the warhead to run the engines: the farther you go the lower the yield remaining in your warhead.
If you fire, move away, and your target moves directly on top of the missile before it has a chance to move, that "range zero" hit will do the same damage as a range one hit.

Multiplexing determines the number of targets the ship can attack in one combat turn using any or all of its weapons. Auto-firing Point-defense weapons do not count towards this limit, although manually firing point defense at, say a stack of sats will count against your multiplexing.