“Congress may have intervened..’
On what legal basis could Congress have intervened? When the Federal Government tried to order the State of Texas to review the case the US Supreme Court ruled that the Executive Branch of the Federal Government had no jurisdiction in the matter. They ruled it was a matter for the Texas courts, legislature to handle. Additionally, the Governor does not have the authority to commute his sentance, Texas has a state board of pardons that has that authority.
Treaties take precedence over state rights.
Here is a legal analysis -
http://www.legalweekblogs.com/legalvillage/2008/03/medellin_case_shows_theres_no.html