Undead Night Elves are NOT going to return to the Alliance

[Amadis-draka (Amadis)]

@Helena-wyrmrest-accord

Where was it confirmed Calia was not joining the Horde?

Depends on your interpretation of a translation of an interview done in Chinese:

@Namira-skeram

Q2. Will Calia Menethil and Derek Proudmoore join the Horde in the upcoming story?
A: This is a complicated issue. Calia is very sympathetic towards the situation and condition of the Forsaken, but she will not join the Horde because of this. Additionally, the number of Forsaken who may welcome their presence is not fixed.