Yes, the backsplash follows the countertop...as it should. It's tricky in an L, and when the slab is this strongly patterned. If it bothers you, I would do a calm tile in stead and keep the counter.

To my eyes one wall of the backsplash is horizontal and the other wall is vertical.

It looks to me like they decided to have the pattern flow in a single direction, L to R. Not oriented along slab length. There’s no wrong or right, IMO, on a choice like that, and if you didn’t specify ahead of time or discuss layout I don’t think you can really complain. Overall it’s beautiful stone and you will get used to the orientation. Enjoy it!

You should have worked this out in advance of the cutting and installation but I would have had the backsplash running horizontally throughout. IMO it makes more sense.

Yes they are correct. The stone mason did a beautiful matching job. Look closely at all four transitions. The grain detail flowing from seam to seam is as perfect as you can get with that pattern. I love that cool tone granite with the warm wood. Very nice.