For help with Mexican laws, you will have to find someone who is familiar with Mexican laws. Given that WA is quite a way from Mexico, most WA attorneys likely would not know Mexican laws offhand.
However, if you are both in WA now, there may be a simple way to fix. You can look into getting married in WA. The marriage procedures in WA are relatively simple: apply for the marriage license, wait 3 days, get married within the specified period. Your local courts likely can perform your marriage ceremony for you. There is no legal requirement for a fancy wedding.
If you or your husband gained immigration benefits because of the Mexican marriage, you should check if the alleged invalidity of that marriage affects the immigration status.
You review your facts and options with an attorney.
You are correct that in most Mexican states you may not remarry unless a year has passed after your divorce. In some states this rule applies only to women. Why? They want to make sure any child the woman has was not a child of the former marriage. I know this doesn't really prove anything but this is how it works. So you are not legally married and you may wish to remarry in Washington as the first attorney suggested.
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