1.) Titan, Twintania, T6 boss, T9 boss (fairly certain since he supposedly has a "Death Sentence")
2.) Whenever you need them. Low priority ones like Bloodbath, Foresight, and Featherfoot/Awareness can be used on cooldown. Vengeance when you expect to take a lot of damage soon. Thrill + Conv to get super pre heals/Stoneskins/Lustrates.
3.) Best aggro opener is Infuriate -> Unchained -> Tomahawk -> Berserk -> BB combo. You can also use Infuriate -> Tomahawk -> Steel Cyclone -> Storm's Path if you need SP up asap. You can macro a message on Berserk to alert your healers to Esuna the Pacification off you, or just let it rock and get some TP back. If Unchained and Infuriate are up again, that's when you'll want to use Unchained again - best paired with Berserk.
4.) Like all cooldowns, unless you need to combine two Right This Second, you should pop each one in between skills. So Heavy Swing -> Foresight -> Skull Sunder -> Bloodbath -> Butcher's Block.
5.) Nah, sometimes you'll need to use them separately. Not a good idea to combine them in a macro.
6.) If you don't need to use your stacks, don't use them. The crit rate increase is pretty great, so you'll only want to use IB when you NEED to. If damage is too high, crit shouldn't be your priority. Infuriate is used at the start, and then again whenever it's up and you've just used 5 stacks (or need 5 stacks NOW).
7.) Foresight + Awareness. Big huge pulls = Vengeance or Thrill + Convalescence.