Multiple instances of double strike on the same creature are redundant.

An Aura spell that has an illegal target when it tries to resolve doesn't resolve and is instead put into its owner's graveyard. It doesn't enter the battlefield, so constellation abilities don't trigger.

A constellation ability triggers whenever an enchantment enters the battlefield under your control for any reason. Enchantments with other card types, such as enchantment creatures, will also cause constellation abilities to trigger.