Reactivating the mission works like this:
1. You pay a credit for every 10 mission points the mission is worth, rounded up.
2. You complete the mission as normal.
3. You earn the mission points + 10 clintz per mission point. You do NOT earn the Ld or any of the missions' original rewards.
Example: A15MP mission costs 2 credits and rewards you with 150 clintz.
This feature was recently added for Mission Point obsessed players and I assume it also works for NB missions.
Like before, a LD character is temporarly become availibe, provided you complete the associated missions. If you already have the Legendary Charecter, then compleating the LD missions will give you 10 credits. You can pay to extend the mission's due date (but since I never had to do it, I cannot help you there).