This is where it gets difficult. If Cheshire was working with someone, I would say Satori. However, she wouldn't turn her back on him to tamper with it.

Let us say someone did notice Cheshire attempting to enact his plan. I am thinking it's possible that individual conferred with another to cause it to backfire. Therefore, if Lucy was an accomplice to that someone... that someone could very well be Clover Field.

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I realize you do not wish to hear this. But I am willing to hear who you think it is as well.