
Continuity mistake: The tyres on the crotch rocket motorcycles they were riding kept changing from knobbly tyres to road slicks, back and forth depending on what terrain they were on. Easiest to see when Ethan skids on the road to avoid the white van - clear shot of slick tyres - then about 30 seconds later they're on the dirt, he skids into the old car and you can easily see that he's got off-road tyres front and back.
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.
Quantom X ★