The tire wear sounds about right.
The tires on these cars are softer material than more traditional commuter cars. For me, my rear tires wear out before the fronts. I have gotten 20k miles out of a set before, probably average something in the 15k to 20k mile range. I have just over 100k miles, and 6 sets sounds about right. If you track the car, the tires wear out faster, obviously.
I do not have a nav, so can not comment.
I never use the back seats either, so I can't speak to the seat cleaning, but I assume you are talking about the plastic bucket of the seat? What color are your seats there? If black, might want to try "back to black" from Mothers.