Sucks that Florence is kind of dead on Sundays. IŽd sleep in Florence because I love it. Check the train schedules on trenitalia make sure they have a few options for morning departures.
I heard it is dangerous that late but it depends if you are traveling alone and your sex. I would do it as an adventurous 22 yr old male.... pshh.
Def. hit up Boboli Gardens and Piazzale Michelangelo for the sunset. I doubt these restaurants open Sunday and specially now in August everyone goes on vacation. Try Il Pizzaiolo (best pizza I had in Italy over 2 months) or Da Il Latini for fiorentine steak.