He twice mentioned the car is garage kept at work and at home.

Most municipal tap water in US is hard to semi-hard. Most municipal tap water in US is fluoridated at the treatment facility.

Neither should make a difference in cleaning leather.

I know, I was kidding. :)

The dealer can blame it on the excess fluoride for your leather deterioration. That's why I only use water packets I buy from Whole Foods.

Not yet. I clean and condition my seats once a month though.

Slightly OT: Never treated a Q5's leather seats aside from a damp cloth maybe once a year.
No leather issues whatsoever in four years.

Same here. Nappa leather on my previous 3 S4's. Never used anything more than a damp cloth unless there was a significant splotch of dirt or food, then it was a spritz of windex on a paper towel followed by the damp rag. Every car had 5 years and over 100K miles when I traded in and the leather always looked almost showroom new with some minor wrinkles on the side bolster and a slight sag on the drivers seat bottom.

The stitching on the seat is clearly abraded where the leather is also worn. Stitching isn't going to do that unless it's being rubbed, probably by something more than just other fabric - is there some kind of common thing you and/or your wife wear that has hard edges that would contact that area? (Eg: rivets, buckles, purse strap hardware, etc.)

Also the leather in other areas is not behaving this way.

The dealer may appease you somehow if they want to be nice, but it's clearly wear due to the user.

Not really, just the usual contact with my pants getting in and out of the car, and I also try to avoid rubbing against it as much as I can, but can't always since it sticks out so much. Once in the car the only other thing that would remotely rub is the seat belt, but don't think it does.

The area also seems to have lost it's sheen a bit and seem duller that the other leather in the car, I think the protective coating has rubbed off, which is most likely due to the contact of me getting in and out of the car, but the car only has 18k miles, so it's obviously more than just wear due to the user, but guess I'll never know. I'll take it to the dealer soon for the 20k service and see what they say.

Not good enough. You should use Acqua di Cristallo Tributo a Modigliani, Fiji version only. Pocket lint price of $60K per 750mL :)

https://www.therichest.com/expensive...-in-the-world/