1) MightyTeapot has a good combat video, maybe more than one.
2) I went through almost the whole story with my friend, it was fun. Most unlocks are account bound, having alt accounts is unnecessary in my opinion. Alt characters are a must for endgame though. Leveling them takes a short amount of time thanks to Tomes of Knowledge (if I recall correctly).
3) First is story and getting to level 80, get exotic gear, then go through expansion/LW stories and unlock gliding and mounts. You can work on masteries, getting max crafting, map completion, maxing fractal levels, achievement hunting, etc
4 and 5 depend a lot on your pacing. I'd say use exp boosters. Save item/crafting boosters for leveling up your crafting.
6) Definitely get one character to level 80 by hand and play it a bit. Then decide if you want to get them.