1. They get attacked (which is as it should be, but I run magma dragon with Husk and Duchess and Husk gets targeted after his provoke).
2. Elves or Demonspawn but probably demonspawn. This one is a bit dependent of course on how this was set up. There is a lot of rock/paper/scissors in arena. That being said, speed wins for the most part and speed plus buff strip definitely wins. There is a reason that all of the plat guys who have super tanky triple revive defensive teams are still whaling out and auto-farming 24/3 during the speed event. Those two factions have the best speed auras in the game.
3. Maybe. The answer should be no but it is actually skill dependent. For instance, I mentioned Husk previously and for reasons known only to the greatest thinkers of our age, Husk has an ability based on attack. For all practical purposes though, no. Any HP or Def damage dealer will be doing damage based on defense or HP and not based on attack so attack down doesn't do anything. It is exactly the same thing as how defense down doesn't make trunda hit less hard.
4. Total HP - So you take what is on the stats page and then modify it by the aura if there is an HP aura. So reducing the max HP would effect it.