Can a player lie about the contents of her library during Slaughter Games?
Thinking about this further… if it's discovered later that there is another Thoughtseize, we'd be looking at a GRV/FtMGS at that later point, I think.
If after some investigation, Nora just forgot about the 4th copy, then it seems we're done here.
But if she knew she had 4 and let her opponent knowingly commit the GRV, then it seems we're in UC-Cheating territory: “notices an offense committed in his or her (or a teammate's) match and does not call attention to it”. By saying “exactly 3”, I'd have a hard time believing she didn't notice. So the question turns to whether she knew that failing to complete the resolution is illegal, and I'd have a fairly hard time believing she didn't know that either.
So in terms of technical application of the IPG, “Neither a CPV nor cheating” seems easy enough. But if someone is asking us about that scenario, I'd think we'd want to explore the potential results of that choice with them.