You talk like the Horde is going to be weakened by this betrayal of Garrosh. The writings on the wall, brother. The small faction of horde that side with Garrosh leading up to the final battle will be DWARFED by the true horde, The New Horde, the Horde that will not stand for Alliance intrusion. Canonically these raids have two win conditions: Ally side or Horde side and that of course for gameplay mechanics means if you do the raid Ally side expect a slightly different experience than one playing on Horde side. Look at Deathbringer Saurfang back in ICC for an example of this.
In either scenario - the band of Horde representing the new Horde at the behest of Voljin etc go kill the insane leader Garrosh presumably freeing him of his sha corruption because like blizzard is so original
- the alliance storm orgrimmar and pretty much do the exact thing but the true horde, the new horde, says 'great, now gtfo before we fuck you up.' you honestly dont expect to keep a foothold in orgrimmar as alliance do you, the horde are still playable characters rofl