Kotoko snuck in during all the prisoner's voice dramas, she is also incredibly perceptive since she had been researching criminals for a while + studied law for a bit
There's a gray area on if she was actually able to watch the mvs or not because we don't actually know what the contraption that extracts them looks like or how it works completely. Even if she knows the entire story it doesn't really matter to her though because she believes the end justifies the means