1. Arcane is currently best for raiding. However, it is being nerfed in upcoming Patch 5.2. So in a month arcane might no longer be best.
2. Cast and stand in your Rune of Power, cast bomb spell, spam AB until 6 stacks then cast AM on proc. If AM doesn't proc, continue casting AB fishing for procs. If mana starts to go down too fast, swap in a few scorches in between ABs. Refresh bomb as necessary. Goal is to keep your 6-stack fully-stacked the entire fight and to keep mana around 90% to gain max benefit from Mastery. Use AP and mana gem either on cooldown, in conjunction with trinket or enchant procs, or during burn phases.
Alter Time can add to your buff durations. Hit it when you have Time Warp, 2 AM procs, AP, trinket or enchant buffs, etc. After hitting it, cast both AMs and (if more than 1 target) ABar. When Alter Time times out 4 seconds after casting, you'll get the 2 AM procs proc and your full AB debuff stack back. You'll also get a few seconds added to the duration of your short-term buffs.
3. Troll is the best dps boost of all the races for mage. Tailoring and enchanting are also solid choices.
4. Archaeology is not my thing. Spent hours working on it and only got to around 120 skill. Staring at a wall watching paint dry is more interesting than archaelogy in WoW. That said, if you can stand it, at level 90, you can craft an item level 476 bind to account epic staff that can give you a head-start in raiding. Note however, that it takes a month to make, and better weapons drop in easily pugged LFR raids.
5. Stat order is: Int > hit to cap > mastery > haste > crit. Point-for-point Int is worth more than any other stat. But it is not worth more than 2 point of your highest secondary stats. Blizzard changed the way gems are itemized from when you last played. Nowadays, 1 point of int on gems comes at the cost of 2 points of a secondary stat. Because of this you should actually now AVOID int gems. Only use them if absolutely needed for a socket bonus. And even then you should never use pure int gems (exception for JCs and their two self-only bind on pickup gems).
NOTE: Some arcane mages, including myself, prefer to stack haste over mastery. Haste gives less theoretical dps than mastery, but it gives faster stacking and makes the mage feel more maneuverable.
6. The cap for hit is 15%. This is down from 17% when you last played. I'm not sure how much hit rating you need at 90 to get to 15%, but it should be easy to tell looking at your character screen.
NOTE: It is FAR easier to reach the hit cap now than it was back when you last played. In MoP, Expertise counts as hit for mages. You can add additional hit to items with hit on them, by reforging their other secondary stat into Expertise.
7. See answer to #5. Yes, int is the best stat, but because of point allocation on gems, you should avoid int on your gems as much as possible.
8. Mastery is theoretically the top secondary dps stat after hit for mages. Since your mana should never drop below 85% or so, you should always benefit from high mastery.
9. Best raiding enchants? Usually pretty obvious. Jade Spirit on weapon. For everything else just check my armory. If an int enchant is available go for it. For items without int enchants refer to the stat priority list.
10. Gem priorties:
Yellow slots: 320 mastery (or haste if you go for a haste build)
Red slots: 80 int, 160 mastery (or haste if you go for a hste build)
Blue slots: 320 hit.
Head socket: Burning Primal Diamond
Weapon Sha gem socket: 500 int