Think about it. If this woman murders someone (Let's make him a U.S. citizen) while the aircraft is on final approach to Dallas, and is taken into custody at the gate, did she commit murder in the U.S. or Mexico? Does the Dallas D.A. simply say, "Can't do anything about it. Let's get coffee?"
Here's another hypothetical: Fed up with the corruption of the Daley Administration in Chicago, I decide to defect to Jamaica. Finding no Jamaican embassy in the city, I travel to O'Hare, board an Air Jamaica flight, and request political asylum while it is still at the gate. Legally valid, or not?
This is a red herring. It's a Federal crime to murder someone on a flight bound for or leaving the US, on a US-flagged aircraft, or for that matter to murder a US citizen anywhere in the world. It would not be handled by the local yokels, and it would most definitely be prosecuted by the Feds no matter how loudly the Mexicans squealed about their sovereignty.
-ccm