I don't think having the advice of the Mexican Consulate would have made any difference in this guy's trial nor guilty verdict. From what I've read, there does not seem to be a whole lot of room for doubt about what occurred nor who the perpetrator of the crime was.
Speaking for myself, having someone from the Mexican Consulate there to "help" the defense, particularly in the case of someone who has lived here in the US since the age of two or so, would probably make me if anything less likely to have a favorable opinion of the defendant.
That not only applies to killing. If you wrong someone in Texas, they will wrong you back. That is guaranteed. Personally got caught between two groups of Texans. And had no choice but to wrong one of them, and yes, they got me back. No hard feelings from me. It was kinda out of my hands.
And that perb spent over the last 20 years of his life in America. So why on earth would he need a Mexican lawyer ? This world is nuts.