First, how did you get scammed by A when he doesn't ship to Mexico? Hoping for clarification.
Second, I'm in Mexico too, and to me your idea is a good one. Here's my reasoning why it is:
I had a cab driver take me around to different vet pharmacies and go in and try to buy N for me, and offer a bribe. They all said not without a prescription.
I went to a pharmacy doctor to get a diazepam prescription. First time no problem, the entered it in the system, I went into the pharmacy and bought it. But it was a one-time deal. So the second time, I saw a different doctor and gave him a fake name so I he wouldn't see I'd gotten it before. He left the room and came back with a photocopied blank prescription form and said it would cost an additional hundred pesos (five dollars) for the "favor" because it's a controlled substance. He did not enter it in the system.
If I were to do what you're suggesting, I would make it from a vet in a different Mexican state or from the US. I'd research veterinary license numbers in Mexico and whether they put them on prescriptions. I hope you'll post if you do this!
P.S. The doctor who gave me the "favor" -- the paper was clearly a copy of a copy, like it's been copied many times not from the original, but from copies. It was half a sheet of paper, and was cut with scissors. It did not at all look neat and professional, which is in keeping with Mexico.