And so on. Something can be given an ability later on, of course. Legal but unlikely, since Interdict is an interrupt. No, since it doesn't say that it does. While fizzling against one target doesn't cause a spell or ability to fizzle against its other targets, you can't exchange only one thing.
Cards count as copies of themselves.
Its first two abilities are written as being triggered, but they won't do much good if they take effect just after attackers are declared. Sure, and you'll skip that many of your future turns. Only to the extent that creatures which are required to attack in turn require their controller to declare an attack instead of letting the main phase end. The target opponent chooses that card, and you remove it from the game. Restrictions on how many targets may be chosen apply only when those targets are actually chosen.