TOPIC:

Is it possible to select a route different from the one which Nav selects between A and B ?

The Nav has options for Fastest/Shortest/Eco and so on routes, all selectable in Route options. If you don't like them, you could add a Waypoint. It's a point you want to go through and by this, alter the route. Finally, you could save your route in Address book, so you don't have to "dial" it again every time.
Another way is to detour incoming sections. If I'm not mistaken, the menu is called "Avoidances". You can specify what distance ahead you want to avoid. Everything is displayed graphically on screen, so you see what are the proposals and how different are they from the original route.