My party of 6 will stay in Chenonceau and will have about 2 days to see the Loire. Please answer a few questions to help me plan our time: What are your favorite 3 or 4 chateaus? What are your favorite 2 towns? Is the idea of doing chateau "drive-bys" realistic... that is, are many of them visible from the road without having to pay entry? How much time should we expect to spend at each chateau we plan to tour? (We will be driving from Brittany and then on to Burgundy so would plan to see the western Loire on the way in and the eastern Loire on the way out.)
My two favorite chateaux are Chenonceau and Chambord. But I like all of them. I'd say 2-3 hrs for each. But it's totally subjective. The best part of Chenonceau for me are the grounds. They also have a nice cafe. I'd been many times for several hours and always hated leaving so, on our last trip I planned one day to spend the entire day there. From the time they opened until they closed, and it was heaven for me.
My favorite town is Amboise. Leonardo di Vinci's house is there and well worth a couple hours.
Yes, many are visible from the road. Many more you can walk up to and around and only pay if you go inside. Chenonceau you do have to pay before you enter the grounds and walk far enough before you can see it. I'm sure there's some surreptitious way to get there without paying but I'd just pay and go in, as I said, it's my favorite.
The more you know of the history before you go, the more you will enjoy them.
Favorite chateaux - 1. Chenonceau, 2. Villandry (mostly for the gardens), 3. Chambord
Favorite towns - 1. Amboise, for the chateau, Leonardo's home Clos Luce and just the ambiance of the town, 2. I stayed in Azay le Rideau and really liked it. 3. Blois for a little bit bigger town.
Can't address your other questions.