Questions on Unique Summons

[Snacktime]

and a couple other noob questions. thanks in advance.

1) If all of a particular unique summon have been summoned, will I still be able to attempt to cast the summon or will I be notified that the summon is unavailable (sort of like with unique item forge spells)? Or will I just blow my gems?

2) If the unique summon dies, can someone else now summon it, or is it gone forever?

3) What does the note about one of the 3 Elemental Kings being corrupted by blood magic mean?

4) What happens if two mages cast the same unique summon spell (or unique item forge) on the same turn? Does one of them get it and the other blow the gems? Who wins?

5) unrelated to unique summons: let's say I order a commander to move to a different territory. if someone attacks the commander with a spell like Earth Attack, but my commander survives, will my commander then complete his move? It appears to me that he won't, because this just happened to me - my commander survived the Earth Attack but was then slaughtered by the invading army instead of moving away as he was ordered. What's the deal with that?