As an aside, this is a bit bothersome and may show some biases on your side. This was completely irrelevant to how you were handled. There are US-born people that have accents (almost all the people in Charleston, SC for example )

Mexico is a nightmare for regulations. If for whatever reason, they deem the papers "incorrect" they put you on the next plane back and fine the airline $10,000. This has happened to me personally.

Even the consulate website does not keep track of the changing rules. I think they do this once in a while when they are short of money. Gate agents that allow this to happen may be penalized by the airline.

The security check that you had to go through happens to people all the time. There may have been a better way to handle this although the "redcoat" seems to have taken care of it in the easiest way possible. I am just wondering if your perception of the agent's ethnicity is coloring your perception of the incident and your attitude towards the agent.

For the future, I strongly recommend getting a passport for international travel as someone suggested above.