Sylvanas is as intelligent as she is powerful. She is always one step above everyone else even when things don't go exactly as planned and that's because she always has a backup plan. She is the only one to get everything she wanted so far got gotten away with it. She caused way more damage than Garrosh beating him in his own game and still got away with it.
She knew N'zoth would wasted one way or another. She wanted Azshara to finish the Heroes but then kill N'zoth but that didn't happen. Backup plan 1 comes into play where the champions of Azeroth with the Heart of Azeroth would kill N'zoth and we did. Had we failed she'll still have yet another backup plan where she'll personally go kill N'zoth herself.
She knew Godfrey has a chance to betray her but she wasn't worried because she knew her servants will resurrect her in case it happens.
The war with the Worgen it doesn't matters if she wins or loses the campaign. Winning would be a plus but still the most important thing is the hungering darkness so she benefits no matter what.
Deathwing managed to destroy the world pillar completely and turn the planet into a wasteland. No problem all the deaths will fuel her partner and she'll kill Deathwing and his masters herself.
The Legion managed to win then no problem Argus would had wiped out most of reality empowering her master even more and she'll be unstoppable at that point. She'll kill the legion and probably even Sargeras with that much souls.
The war in Pandaria and Garrosh's True Horde. She was fine with it as every side gets casualties. Had we failed to stop Garrosh then no problem cause her backup plan would be her killing Garrosh herself.
Tricking the Alliance and Horde rebels to fight with her forsaken loyalist in Origimmar but Saurfang somehow made her expose herself. No problem because losing the Horde at this point doesn't matter anymore cause she has already gotten more souls.
There are many more examples and as you can all see, She has backup upon backup plans. She will outplay anyone that tries to mess with her. Everyone's been playing checkers but she's playing chess.