1.) I went immortal because I liked being able to tank flashpoints and heroics.
2.) Defensive stance since the mitigation benefits against elites or strong packs outweigh the slight dps increase of the neutral stance.
3.) Vette until the mid thirties to low forties then I switched to Quinn when I had the gear to make him a good healer. At my current level (47) I can solo all of the same level 2+ heroics, anything in the world (without ever having to meditate), and duo all of the 4 person heroics.
4.) Um, all of them really. Pretty much all of my abilities get used when they are off of cooldown with priority going to scream and immortals 41 point talent. You're baseline rage dump is the same as it is at level one. Always hit retaliate when it procs.
Honestly the class is easy mode to level once you hit 30 and just slightly slow before that. Use vette until 35 to keep up decent pack clearing times and then quinn after once you get some decent AoE youself with Smash and the other AoE ability. I can solo open world champs without ever dropping below 90% health and not using any cooldowns. Granted other classes can as well, and faster.. but they also don't get instant grouped for FPs or heroics unless they are a healer.
edit:
I also have a 35 powertech and a 40 healing spec merc. The jugg is by far the easiest to level for me.