My daughter is getting "'married"--a spiritual ceremony in Playa del Carmen. After looking at all the requirements and restrictions, we all decided that it would be better if she got married in a civil ceremony in the States. Then you know that you are realloy married! I recommend you do the same (and it's a lot cheaper).
The civil ceremony seems so complicated in Mexico. Think about just doing it in the States right before you have the reception. Then you know it's legal.