It’s probably accurate to say that they Alliance didn’t actually control Quel’Danas, though it ultimately doesn’t change the sentiment I expressed above – because the Alliance most certainly dominated the Sunwell Plateau, and would’ve surely been able to defend their position from any incursion from either the Blood Elves or the Horde.
Instead, the Alliance decided the Sunwell belonged to Quel’thalas. They didn’t even attempt to piggyback off of Velen’s revitalization of the Sunwell as a diplomatic bargaining chip… and, at this point in time, the Blood Elves probably would’ve agreed to virtually any request if it meant they’d have uncontested and unfettered access to the renewed Sunwell. /shrug
They did.
The point I was ultimately attempting to make is that the Alliance had all of the power in the post-Sunwell Plateau period, and yet elected to not utilize it at all – like I said, had they been asked and reasoned with, it would’ve been entirely in keeping with the Blood Elves characterization for them to immediately abandon the Horde in favor of joining the group who had just prevented the Sunwell from being destroyed permanently, imbued it with the Light making it even more beneficial to the entire population, and who asked for nothing in doing so.