The Alliance aren't going to be liberating the city, otherwise there'd be nothing for horde players to do. I don't see it being a full on Alliance invasion, instead I'd expect some kind of infiltration mission.
More importantly, I don't think the Alliance would be wise to invade Orgrimmar and claim it when the full might of the Horde is facing inwards. There is going to be some kind of power vacuum within the Horde post-Garrosh, crushing the Alliance interlopers would suit some members just fine (Sylvannas, I'm looking at you). And if we're going by logic, why would the Alliance bother to hand back even half of the city? Why wouldn't Varian and Tyrande seize the opportunity to wipe out the settlement and many of its inhabitants? The only answer is that they couldn't manage that because, as I said, the entire might of the Horde will be facing inwards.