Defense position monsters will get banished, preventing them from activating their effects that usually happen if they get sent to the graveyard.
Monsters have to be in DEF position to resolve this effect.
It can only work if the attacking monster still go through with the attack.
Using Windstorm of Etaqua will not work with this card since the attacking monster will get switched to defense. You can use cards like Sphere Kuriboh to target specific monsters, but usually Tragedy is a better option.
Not really: you're not allowed to activate Dark Mirror Force if your opponent has no defense position monsters, so you can't do "I activate Dark Mirror Force and chain Etaqua". You're far better off with Tragedy.
Old school yu gi oh PC game, destroy all atk position monsters when opponent's attack.. it's different..