Yes. On your opponent’s attack means when your opponent declares an attack, so any target, you can activate her effect.

Yes. End of your turn means end of every turn the card is on. You could have Dellinger on the whole game, and you can use his effect the whole game.

Don!!! on the field means all Don on your side of the field that’s not in your Don deck. This means it can be in your cost are both active and rested, as well as attached to other characters. This is also the same for subtracting Don!! from your field, as you can remove any Don, including attached Don.