Other mistake: Ambrose smacks a stone on Ethan's forehead. No blood nor even a bruise appears seconds later when Ethan is kicked in the face and we get a close-up of him.
Other mistake: Luther tells Ethan that he will only have 40 seconds for the louvres to remain open for him enter the ventilation shaft at Biocyte before the alarm goes off. But the warning message on Luther's computer says that the limit is 20 seconds. (01:06:30 - 01:09:10)
Answer: Ethan was the most probable candidate. Remember that Ambrose and Ethan had specifically worked together a few times with IMF in the past, as stated in the film, with them body doubling each other. Once Ambrose had his suspicious raised by the envelope, he decided to test Nyah. So showing up to her with Ethan's face would indicate if she recognized Ethan, or thought she was being approached by a total stranger at night.
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