If an opponent casts Falling Star targeting my 4-might unit twice, and I use Stupefy to reduce it to 3 might (causing the first instance to kill it), does the second instance of Falling Star redirect to hit the opponent's Darius?

Ruling: No, the second instance of Falling Star does not redirect to Darius. Both targets for Falling Star are selected at the same time when the spell is cast, so both instances are targeting the original unit even if it dies before the second instance resolves.