A couple of questions:
1. Can this be easily done in one day?
2. If the weather is wet, is Byway 12 an easy road to drive on?
As I am from Australia, driving on the 'wrong' side of the road is a bit of a challenge, a wet, slippery road will not help matters.
For your information, I planned by trip based on input from the TripAdvisor community and it has been a very pleasurable experience.