I did hear the call/interview of the witness and I have to say he sounds like thousands of other Hispanic Texans. I didn't notice anything odd about the accent. In fact, he sounded suspiciously like my neighbor 2 houses down.
That neighbor was born and mostly raised in Mexico, but spent about half his childhoood in the US with relatives and has lived in the US since adulthood, even being in the US Army. There are various accents and that's just one of them. As for not not wanting his face shown.....No one in his right mind would want his face shown or his ID given if he believes this story to be true. He might as well go ahead and pre-plan his funeral and pay up the life insurance policy.
Still haven't had time to assess the wife. Sorry, but I'm busy planning a party. 
But let me make it clear that it would be in the best interest of Mexican law enforcement to NEVER identify any shooters or even find a body. They do not want an international incident. It's bad enough that the body count from the drug cartels on the Mexican side of the border is staggering, but most Americans don't really hear about it and aren't that upset about it. (Just those of us on the border who are scared because we hear about it all the time.) But this incident, if true, would bring all sorts of unwanted attention to the fact that Mexico is helpless against the cartels. It would be an embarrassment all the way up to the Mexican president's office. And that's just the PR angle.
There's a good chance at least some of local law enforcement are on the paytroll of the cartels. Good heavens, at the end of August 1/10 of Mexico's
FEDERAL police force was fired for "not doing their work or for corruption." Translation: Tied to cartels. Can you imagine if 1/10 of our federal agents were fired in one day for corruption? And if Mexican law enforcement isn't being paid off, they still don't want to point the finger at the cartels because the finger that points back will probably be holding an AK47. Police are regularly murdered down there, and not in the line of duty. BTW, "kidnapping" doesn't work the way we tend to think of it. If someone is actually taken, the chances of ever seeing them alive are slim and none....closer to none.
I did catch the US side local police chief. (?) He essentially said that there was no way they were going to take their lives in their hands and cross over the border to investigate this. It was too dangerous and he feared a gun battle. No duh. Smart guy.
All I am trying to say to those who cannot fathom why a police force would cast doubt on someone's story unless they truly doubted that story is.......Because they prefer to stay alive another day. If they weigh their own lives and that of their wives, children, parents, etc. against that of an American tourist, it's not hard to see why it would be in their best interest to make the wife look less than credible.
If the wife did do in her DH, then she may be one smart cookie because she has a better grasp of the politics than most. The Mexicans will cast doubt and never search in earnest (or at least delay as long as possible) for fear of actually finding evidence a cartel was responsible. If that happened, they'd be in a real bind. The US government will never call the Mexicans on it because goodness knows we can't offend them. The US can't really send our own people to investigate because it's MEXICO and even if we could, they don't want to go because the risk of death is too dang high. Basically, too many people who have power and matter just want this story to GO AWAY. By any means necessary.
