My recollection is that most tourist offices will be able to give you maps of VTT (
Vélo
Tout
Terrain) routes. The routes themselves are pretty well signposted (as far as I could tell, I was road riding whilst I was there, so only saw the beginnings and ends of the off road tracks).
In Normandy at least, there's a shortish post with a coloured symbol (which indicates the level of difficulty) indicating the path.
I can't answer your other questions, but consider contacting the tourist office of the place you're heading for, and perhaps the Federation Francaise de Cyclotourisme (
http://www.ffct.org/)