They should. I mean, unless their information network is utterly and completely inept, it would be practically impossible for them not to know that Vol'jin is dead and Sylvanas is the new warchief. I mean, hell, as the horde player, you are informed almost immediately after returning from the Broken Isles assault scenario that the Horde is aware that Vairan is dead and that the alliance is blaming us for it and rattling sabers in our direction. And this is before Vol'jin even kicks the bucket. If the Horde can get that kind of battlefield intelligence in that short of time, the Alliance has no excuse for not knowing Sylvanas is inducted as the new warchief in a massive ceremony shortly there after.