Eh. A lot of this is addressed in-universe, and respectfully, some of it's just wrong.

1: All barrier spells in the game are instantaneous, whether they're player or NPC abilities. Thalyssra conjures a similar barrier during the Stormwind extraction scenario and is able to hold it against dozens of attackers.

2: This isn't unique to Jaina. Malfurion shares the archdruid's lunarwing form with players, and for all I know there are other examples running around. I think Blizzard's just trying to point out that it's only people at the absolute pinnacle of their classes (e.g., major lore figures and the PC) who'd have these mounts.

3: By that rationale, every raid boss is a Mary Sue. If you want to consider game mechanics an inescapable part of lore, then Jaina's actually considered a fun fight but undertuned at the moment.

4: Frost, ice, and water magic is her thing. This is probably the most visible sign of her staff's influence, because just about everything else she does is a souped-up version of basic frost mage abilities. The whole point of being a frost mage is to get away from a fight gone bad. If you've ever been a melee player trying to chase down a frost mage in battlegrounds or arena, you'll know how maddening it is to kill one who's any good.

5: If you play through the Alliance side of the raid, you see Jaina unconscious on the deck and being attended to by a healer immediately after the fight.

6: Azshara is canonically much more powerful than Jaina and is reputed to be the most powerful mortal magic-wielder of all time. We don't actually know what's going to happen in the next raid yet, but I would be very surprised if we wind up offing her in a mid-expansion raid. Even if we do, Jaina wasn't there to fight to the death; she's there to delay the Horde's pursuit of the Alliance as it withdraws.