This. ^
Like I said earlier in the thread, the only way this works out for Sylvanas is if the Alliance kill Saurfang to get him out of her way and give the honorable part of the Horde a martyr at the same time. Personally, I can't wait for him to come back and challenge her/lop her head off so the Horde can maybe get close to what Thrall had in mind for it.
What's actually hilarious that you believe this could happen and seem to want writing this bad to happen. Please elaborate on why someone impersonating Saurfang wouldn't kill Malfurion. If your answer is "Alliance in disguise", please elaborate why they would keep them in the Stockade? If your answer to that is "to fool the Horde that break in", please tell how the Alliance knew the Horde were going to break in so that the Alliance could fool them and why bother risking the impostor's life for that? It wouldn't be out of the question to put Saurfang, a venerated war hero and leader of the Horde, somewhere more secure than where they throw corrupt nobles and Defias, so the Horde probably wouldn't think it odd he wasn't in there.
Edit: Before you say "they put Talanji and Zul there", Saurfang is a little more important than two trolls from a doomed faction we've been whooping for years now.