In some instances, I could see her attacking her without prior knowledge of how strong she actually is, which could result in her losing the fight. It's difficult to say either way, though, as there are also a similar number of possible instances where she does remain vigilant and decides to end the fight as quickly and effectively as she can.
Since we don't have much in terms of in-verse demonstrations of how she fights, and the moves she chooses to utilize first, all of them are plausible options. I'd argue that, given her intelligence and Ciri being an unknown factor, she would stay back and accrue information about her before engaging. And with the Heart's ability to tell weaknesses and other forms of information about a target, I could see her gathering enough pertinent information to determine how dangerous Ciri is and act accordingly.
As for Mesmerize, I see on the Wiki it says it takes 2 seconds to affect someone, but I don't know if this is, as you said, a gameplay limitation or an actual one. For the sake of fair debate, I am fine with assuming it is an actual limitation (it is demonstrated as one, even if it is never explicitly stated to be one). I'd simply ask, though, how would Ciri address the Void Thrall? Like, would she simply try to attack it (which won't do anything to it, If I recall correctly) or would she, because of experience and instinct, decide to teleport away and avoid looking at it. I don't see why she would have any prior inclination about how the Void Thrall functions, so I don't see her simply not looking at it.
I see her more likely deciding to attack it, which might not work, which would result in her getting mind controlled. It would allow Emily some much needed time to set up the combo I brought up. So, until I see an argument why she wouldn't be affected by the Void Thrall, I have Emily winning via the argument I laid out here and above.