Thanks a lot for the advice. The plan is now to travel from Cedar Point and arrive at Knoebels at 4ish and stay till close, then just have the one full day at Hershey.
The only reason we've added Dorney in is because it is right on the way back to the airport and also because our plane doesn't leave till 9pm, So we will have a fairly large amount of time to kill that day. Plus Talon does look pretty good!
I was a bit worried about the Kings Island part of the trip, as KI shuts at 8PM that day too. What you suggested is a good idea and we will probably do this, as I think Dollywood shuts at 7PM that day anyway.