Book the bus directly through Alsa themselves mate. That's the Spanish national bus company:
Buy bus tickets at the best price. At Alsa we offer bus rental, travel deals, on-board services and information on routes and schedules.
www.alsa.com
Quite a few busses Granada to Sevilla on the day. First bus back is 9am Thurs morning, but there's a train back at 7am for €23.50 on Renfe (national train provider)
https://www.renfe.com/es/en
Failing that, have a look at Bla Bla Car, lifts back from Sevilla to Granada have been popping up all day at weird and wonderful times and will continue to be added over the week.