I wish this forum allowed editing!!! Additions/correction in Italics
From #2 above:
Use a GOAL STATE EVENT on the first flight, e.g. MUST DESTROY[FIRST FLIGHT]=TRUE to set a FLAG (say USER FLAG 1) with the CONDITION that the MUST DESTROY[TARGET]=FALSE
Now set the TAKEOFF action for the Backup flight to USER FLAG 1=TRUE.
This way, if the original flight is destroyed BEFORE hitting its target, the backup flight will launch. If the Target is destroyed, the backup flight will NOT launch, even if the first flight has been destroyed because the condition that the MUST DESTROY[TARGET]=FALSE is not met.