It is formal. Take into consideration that the average temperature for May is between 77 and 99 degrees.
You can either take a taxi, Uber, or rent a car. Valet parking service will be available at the reception.
Weddings in Mexico are usually that long, but do not feel obligated to be there until the very end (though it would be amazing!).
Please refer to the Registry section on this website.
Please, refer to the Things To Do section on this website.
Merida is the safest city in Mexico. However, as any other city, it is suggested to take the usual precautions.