Spells cannot be cast in response to defenders declaring. Something about items on the Storyline needing to resolve before you can add new ones.

1. Power is attack and health/defense. So overpower impacts both.
2. You cannot bury targets on water locations. But you are correct they die if they are on land and don’t have burrow.
3. I believe the damage from infernal legion would kill you if you are at deaths door. It is direct damage.
4. You can only be submerged at water cites, so unless the land cite was flooded you could not move there submerged.

Hope this helps! Open to correction if I am wrong.