@ #1 - My personal preference is Engineering and Jewelcrafting for min/maxing stats. Engineering offers HUGE armor on the gloves and nitro boosts on the boots can be invaluable. Blacksmithing and any other profession that can provide stamina is a suitable alternative. If you're going to be gearing up normally, Blacksmithing may have a greater value, due to you being able to craft your own gear should you need to.
@ #2 - I assume you're talking about reputations, which you should aim for first? I believe Argent Crusade has the helm enchant. Items from the reputation vendors don't last long at all, though. I wouldn't bother too much with them. To be able to queue up for RDF, you should be able to at about 180 average item level. You'll want to be at about 200 average item level before you want to go into FoS & PoS, and be as close to your defense cap (540) as much as possible. The heroic versions of your dungeons, you'll want to be closer to 215-220 average item level before you go into those, and should be fairly easy to obtain once you're able to farm RDF and FoS/PoS.
@ #3 - Correct. 535 defense is the cap for a level 82. 540 is the cap for a level 83 (usually a raid boss).
@ #4 - Harbinger's Bone Band
Wapach's Spaulders of Solidarity
Torta's Oversized Choker OR Burning Skull Pendant
Avool's Sword of Jin <- technically not a tanking weapon, but gives hit and helps a bit with threat when first starting to gear up.
Pillars of Might
The rest is up to personal preference, but I wouldn't buy any crafteds that are under 220 item level.