What kind of heresy is this? The only argument in favor of the claim that Calia's glorious return shouldn't give the Forsaken a 180 turn is that it should give them a 540 degrees one. Anything else is sinful. Blizzard's writers are professionals that are employed in the field and that automatically makes them amazeballs because it's just how things work. So trying to present this development as unbelievable has no support in how things are.
Maybe she's a Paladin now and her resistance aura also boosts resistance to depression.
Oh yes, the post-BfA WoW that praises unification and pushes it onto the factions no matter how hamfisted it is is bound to be nuanced and not straightforward in the least. The Forsaken will for sure oppose Calia for centuries before they buy into her perfect perfection. And if only that potential you mention didn't reek of the human variety.