I notice that since day one the car prefers the right edge of the lane.
Otherwise I use it each way to work and back every day and enjoy taking my hands off the wheel for as much as 3 minutes at a time.
Noticed that as well, though I timed it at 2min exactly hands-off time nag-to-nag. I also notice it refuses to do hands-off lane changes, but if you apply just a hint of torque in the opposite direction of the lane change, it'll still execute the lane change all on its own. I also notice something else about the hands-on nags: it'll nag you any time, reliably, that it thinks it could lose the road. It nags before a lane change, and at least once or twice, I've experienced it doing a bad job finding the new lane such that I preferred to help it out. I've also noticed it nagging when turning into a bend at high speed, the kind where you'd be happy to take it full speed but your passenger might get a little uneasy. I think it nags the moment its lane estimation is at all degraded, or commanded steering torque exceeds a certain threshold.
Is NAV only engaged (green) when the car has reduced it's speed from the setpoint?
No - NAV means navigation-assisted speed. It only goes green when you're at the speed limit for the road, and it means that when the speed limit next changes, your cruise setpoint will change to match. If you're over or under the posted speed limit, it'll maintain your setpoint regardless of what the speed limit does (i.e. you may well blast 66mph into a 50 zone). Not sure how it behaves if you set the "speed limit offset" setting in preferences but I'm assuming that's how you get it to automatically change speeds while deliberately speeding.