As soon as you want! Michael and Annie will be in Austin starting on Thursday.
You won't need one - Austin has uber, Lyft, and RideAustin, plus plenty of activities within walking distance around downtown
https://www.theknot.com/content/wedding-guest-attire-cheat-sheet
It was an... interesting... surprise. Austin is going to be a party!
September? In Austin? Yes. It will be hot.