Want to drive from Santa Flavia to Agrigento on Sunday, 7May. Want to stop for 2 hours in Monreale (will it be open to tour on Sunday?) then onto Segesta to see Temple in 1 hour and end up in Agrigento (2 hour drive).
Is all of this reasonable and what are we missing?
The cathedral in Monreale will have Masses scheduled for Sunday, your visit will have to work around that.
The temple at Segesta will close the entrance one hour before sunset, if not sooner.
Using your timeline, you have about nine hours scheduled that day, with no breaks and no allowance for Sunday traffic.
My favorite spot in Sicily was the ancient hill town of Erice, in the far northwestern corner of the island. I spent two nights there, and especially enjoyed the misty and quiet early mornings and evenings, and the great food, including with some influences from nearby Tunisia. Erice is about one hour from Segesta (certainly my favorite Greek ruins in Sicily),although in a different direction from the other places you mention. Erice also might be more crowded and touristy on a Sunday during the middle of the day, with many local visitors from Palermo and other locations in Sicily.
Both Google maps and viamichelin.com estimate the driving time at close to 5 hours. That's net (probably optimistic), without traffic and without looking for parking in Monreale. It sounds fine as long as you're not planning to sightsee in Agrigento that day. If you get to Monreale during mass, use the time to visit the cloisters first. You may also want to spend a little time wandering around the town (though maybe the shops will be closed on Sunday). At Segesta, you can also take the bus up to see the theatre - as long as you're there, it's well worth the extra time.
What you are missing, most of all, is visiting Selinunte. Of the three temple sights, I liked Selinunte best. Agrigento had considerably more visitors than the others (weekday), and a very good museum. My trip was at the end of April, carpets of wildflowers everywhere.
