Hi oldman and welcome to the forum. If you add your rough location to your profile and your precise model/trim level/wheel size in a signature, you will receive better targeted replies. I suspect you are in the UK since the crabbing only really afflicts RHD 4MATIC vehicles. I will find out all about this if you have a look at the 80-page clonking thread and the FREE all-season tyre swop thread on here.
I suspect MB are playing the normal wear & tear card regarding the Artico. There's a world of difference between doing 6000 miles as a multi-drop local courier and doing 6 return trips from London to Aberdeen. It will be down to you to argue "reasonable" use and even if you get MB to concede, they still won't sanction a warranty claim because it will set a precedent but may repair it as a goodwill gesture. There have never been a successful claim against their much-vaunted 30-year Mobilo corrosion warranty, only ever goodwill payments.