Her methods make more sense if you read the mini-novel of the War of Thorns on their site. Not the whole thing, but the parts in the beginning with her and Saurfang. She's pretty adamant about hitting the Alliance hard enough to get them to fuck off for good. The idea was Stormwind but it was considered impregnable. Its why I think her and Saurfang are still in league, since now he's learned more of Stormwind's weaknesses.
I think she's playing the "villain" by any means to keep up appearances and make Saurfang look good. After I did his scenario on the PTR, its obvious SI:7 is helping him and Anduin is practically encouraging him to retake the Horde.
I think its a scam and Anduin is being suckered. Saurfang and Sylvanas were already playing tricks on them before the War. SI:7 has been spying on the two of them constantly, and they decided to work together to use the Alliance's spies against them. Saurfang talks about honor, but several times mentions he wants the glory of sacking Stormwind.