Have you found these?
Granada 6.45 -tren- 9.18 Ronda
Ronda 13.00 -autobus- 15.00 Sevilla
http://www.turismoderonda.es/informa...ariotrenes.pdf
http://www.turismoderonda.es/informa...oautobuses.pdf
Note they run the summer time in Spain in May.
Then, the current schedules would be changed.
You should check the sunrise/sunset times as well.

Thanks Luisjack.
I did see these times but it seems the buses leave early afternoon or late evening to seville. There's nothing that will get us to seville by early evening, departing Ronda say 1500-1600.
The other concern I have is the lack of left luggage facility at the train station and the very limited space at the bus station. We"lll only have a few hrs at Ronda, and don't want to spend it looking for a storage.
That's why we thought renting a car could give more flexibility. It's almost the same cost too.