This isn't more of a what she is, what she is doing post more of a venting my opinion on it post.
So we have all known for years Sylvanas is batshit insane worse than a goblin brain in an abomination. Yet I would be really aggravated if she killed off Varian or imprisoned Vol'jin. It doesn't seem like her to be that stupid. She is cold and collected, she wouldn't kill Varian in the middle of a battle when she could just leave him to fend for himself. It doesn't seem like the type of thing for her to want to take over the entire horde. She has is and always wants to do what is best for the forsaken and herself.
Yes I do agree the whole BS the BS in Cata with her using the Valk'yr to raise new forsaken is a little strenuous but I saw it more of a "Hey Garrosh Asshat, you keep sending my people against the alliance and getting them killed. I'm gonna need more people." Hence her assault on Gilneas. Kill the humans make undead, bigger force. Oops they are worgen.
To me Sylvanas will always have backup plans for her back up plans. And when it comes down to it. If she went after the warchief throne it would get her and the forsaken killed. If she did kill Varian it would mean retribution and another stupid ass war where she will lose more forsaken and possibly die in the process.
TL;DR Sylvanas wouldn't do the what people think she did because she would be putting herself and the forsaken at risk.