Uber/Lyft/etc. isn't as prolific here, so renting a car is recommended. With that said, Downstream Casino (suggested place to stay) is a ~5 min. drive from the wedding venue.
Two airports fly to Joplin-JLN: Denver-DEN, and Chicago-ORD.
Formal wear is fine. Please make sure to check weather before coming. It can be either warm or cold around this time of the year.