I'm going to grant you that one, but it seems that if it were important, the defense attorney could have brought it up. In fact, the defense attorney could have made the phone call to the consulate himself.
I do wonder if the treaty in question was ever ratified... or if similar to Kyoto and the International Criminal Court it was simply signed by the (previous) president, but then tabled by the Senate, leaving it null and void. It was not necessary to "withdraw" from Kyoto and ICC, for example, because they never went into effect.
If this one is in effect, we should look at the other clauses, to see if and in what way our sovereignty is limited, and if it goes beyond what is reasonable and in our interest, it should certainly be abrogated.
And finally, the whole purpose of a murder trial is to determine guilt, and to deliver justice on behalf of the victim. The step of notifying the consulate would allow the Mexicans to assure themselves that the accused had adequate counsel. I'm not opposed to this. But we should not allow them to politicize the process. If Mexico wants to represent the accused, are they also prepared to make the victims whole?
There are two treaties which come into play here: the 1963 Consular Rights treaty, and the World Court treaty. The Consular Rights treaty is real, was ratified, and is important (because we would like the US Consulate to be informed if US citizens are arrested abroad). But, under usual rules of international law, a violation of that treaty doesn't void a criminal conviction, it just gives the foreign country the right to take diplomatic retaliation against the US.
The World Court traty was also ratified by the US, but with an important reservation: we agreed to accept the Court's jurisdiction only where it didn't interfere with our internal affairs. These orders clearly do that, which is why we're ignoring them.