I have to admit I didn't see that coming until about a chapter or two before the reveal. I usually pride myself in my ability to pick up on some plot twists based on hints authors tend to give you ahead of time. But this one caught me unawares.
A bit ironic too because early on in the book, when "L'Nel" asked Sarek about approaching Spock to be her mate, I thought of T'Pring and wondered what happened to her. But honestly, I just didn't give any thought at that point that she might in fact be T'Pring
I didn't know it was her, though I figured out what she had planned. In keeping with TOS tradition for the criminally insane, she is not killed, but rather sent to a mental health institution.