Agreed with every thing above (Other than the last part.)
I'm pretty sure Alleria will be a heavy point in Sylvanas's character development, as well as perhaps the only driving force in a unified Quel'thalas (as far as Quel'dorei and Sin'dorei are concerned.) I really can't imagine her not coming back and bitchslaping both her sisters. Needless to say, I'm looking forward to the day the day we see her again.
On the Metzen forgeting he said it note. I half agree and half disagree. Several interviews this expansion with Ghost Crawler have said that for the first time ever Blizzard is planing for future expansions ahead of time. So Metzen saying we'll see them next expansion is not really unbelieveable.
However Dave Kosak in a recent interview also said, something about wanting to add Turalyon and Alleria back into the game but they are having issues doing it.
To be honest, I don't know who to believe on that. Dev interviews have been full of mix information and lies that speculation over it seems almost pointless.