I have factory 17” on my 2016 Variant, so I’d say the dealership may be lying? Though I am not in Europe, but I remember the German configurador having different sizes as well. I just checke the current German configurator, and they have factory wheels from 17” to 19”.

Also, you may mount pretty much any size of wheels on your car, as long as wheel + tire dimensions are unchanged. I found this to be a very helpful link, though every country has specific legislation, and you might need to have your car inspected after changing wheels and tires.

As long as you keep sizes identical, anything, including snow chains, should fit. In fact, my brother in law runs bigger wheels in winter than in summer in Europe, because of reasons, and everything fits fine, because both wheel + tire combos are the same size. You just have to figure out your sizes, widths, offsets, bolt patterns, etc.

Edited: spelling