1: Flame Diamonds can be obtained from killing bosses in Mirror dungeons. Bezemut, Ruler and Goddess Silva also drop new boss drops.
2: No conditions required to grind there, If you have 100k hp, 8k pdef and absorb on your weapon you should be good. No point in grinding there though as no one wants the drops anymore.
3: At least 100k hp, absorb on weapon and a high dps class. Pred, Templar and Sage all do well there. Any other class and you will need really good gears to solo.
4: If you can't do Mirror then don't even worry about the Abyss maps yet.
5: Either be a great support or high DPS.
6: Tickets can be obtained from Time jewel boxes and event boxes (currently Savage boxes and Winter boxes)
7: Depends on your view tbh. There are a couple of good GM's, lets just leave it at that.