It's pretty much done at the speed of thought in general. At first she thinks something is going on with Vellcinda and Anduin as at one point when Anduin catches sight of Vellcinda he bows to her from across the field, and Vellcinda had made a request to meet with Anduin as she herself had no living family to meet with (the request was denied by Sylvanas). Because of this her first impression was that this had all been set-up prior, but on seeing more of what was happening she concludes (correctly as we know from the other perspectives) that Calia's revealing herself couldn't have been precipitated by Anduin or Vellcinda as there wouldn't have been a way for them to plan it without her knowledge. She considers staying her hand briefly, but then decides she can't take the chance of renewed "hope" among her subjects and unleashes her Rangers.
The horn was sounded because there was a pretty obvious consternation happening on the field when Calia revealed herself - given the small number of people there and the intense scrutiny of the surveying Priests and Rangers, it was easy to see that *something* was happening that was not otherwise planned. One of the Dark Rangers then flew back to Galen's Fall and informed Sylvanas that the source of the consternation was confirmed to be Calia Menethil, which pretty much immediately got Sylvanas' back up. She rocketed through several emotions all at once (and considered it from all angles as related above) and then ordered her Rangers to commence the massacre. As it is ongoing, when Nathanos rounds on her to ask what she's doing, she explains her thinking on the matter and justifies the decision to him.
Sylvanas was already waiting for the whole thing to go pear-shaped. She thought the entire idea of the Gathering was pointless and a waste of time, but she allowed it because either way it could be of benefit to her. If it was a trap then she could use it to justify the destruction or disbanding of the Desolate Council and imprison (or likely execute) all its members without looking bad, and if it was successful she could use it as a lever to keep them in line in any case, dangling the idea of hope as a carrot to move them how she needed. She didn't expect Calia Menethil to show her or to have to deal with a possible usurper of her throne, however; and that led to executing the "disloyal" Council members and Calia herself pretty much on the spot.
The initial view of the Desolate Council didn't have anything to do with reconnecting with their living families, though; that was all Anduin's idea and doing. What the Desolate Council wanted was the option to die the "Final Death" like Humans do, basically they were suing for the right to die as opposed to Sylvanas' plans to make them immortal. Forsaken existence is a difficult and cursed lot, in their view, and while they were grateful for their second chance at "life" and freedom from the yoke of the Scourge, they wanted to live and die as Humans otherwise would. Sylvanas needs the Forsaken to persist forever (to protect her from her own death), which is what her attempting to enslave Eyir was all about.
But now the story isn't just about her anymore. If Sylvanas becomes a villain, that would also affect Vol'jin's story, because he was the one who vouched for her and placed her in charge. Garrosh was a great villain, especially for Horde players, but his villainy completely ruined Thrall as a character. And at this point it would affect the Horde's story as a whole, turning us into morons who can't pick a competent leader if the whole world depended on it, not to mention the repeat of the "evil Warchief appointed by naive good guy" would be hilariously bad from a storytelling perspective.
So it's kind of weird to see her pull off borderline irredeemable shit like this. I want the Alliance to have some flaws and internal tensions as well, especially with the humans, so I can justify hating them as a Horde player.[/QUOTE]
I don't really know how to reconcile the current Sylvanas with Vol'jin's dying proclamation - but I assume there is *something* yet to be revealed or discovered that will more clearly explain the impetus behind Vol'jin's promotion of Sylvanas. Sylvanas is primarily motivated by fear of her own Final Death, as well as hatred and anger that still exist in her core that has no outlet with Arthas now dead himself. But she still has moments, fleeting ones and rather few in number, where she reveals a part of herself that is worthwhile or worthy of redemption, if you prefer. She might under different circumstances even be a great leader, but I don't know if she can be salvaged given everything she's already done as Warchief (and will do from what we know of BfA's events).