Okay, I think I made a bad assumption. I read it as: Fail Save -> 1D5 Stress -> Immediate Panic Check -> Mind-Controlled if that fails too.
If fully falling under her sway is only possible if something else causes a panic check... well, she could still kill someone else on round one to force it. However, that does now require four separate rolls to all go badly... fail save, then she rolls to hit, rolls high on damage and one-shots your buddy, you fail the panic check...
It's not likely to happen, even under my original assumptions. And you're right that it's "fair" in that they could equally one-shot her. But either of those things could happen quite early on in a perfect storm, and I'm instinctively averse to front-loading decisive rolls in that way. I want to be sure there are going to be at least a couple ebbs and flows of tension before we get to the point that the whole thing could be over on the next roll.