There's just me living in this space. In the 4mths I've lived here so far I haven't had to clean the tiles on back wall but it is dark so maybe I can't see the mess. As for the sink area there's water splashes always on the tiles. The kitchen designer advised me against using the mirror here for that reason.

Mirror splash backs would look lovely in a show home. Mirrors are only stunning when there sparkling clean. They would annoy me if they were constantly needing to be cleaned. I think you would be creating an on going chore for yourself, like Karen Mc, they would drive me crazy. Why don't you have a look at some of these https://au.pinterest.com/GDBully/kitchen-window-splashback/ There are endless choices, I wouldn't be making work for myself. Depends I suppose how long you are going to live there.

Under Australian Consumer Law there are regulations for products to be fit for purpose. "Under the Australian Consumer Law, certain consumer guarantees apply automatically, including that a product must be reasonably fit for any purpose specified by the customer and agreed by the seller. If a product is not fit for a specified purpose, the consumer is entitled to a remedy. The type of remedy depends on whether the problem is major or minor." I would check your paperwork to see if there was any explanation that your selection was not fit for purpose, and I would be seeking a refund and going with someone else, who can do a proper mirrored, toughened glass splashback, such as these guys (or somebody similar in your state): https://precisionshowerscreens.com.au/products/austuff-toughened-mirrors/

As a glazier myself, that advice is correct, however non-toughened mirror is significantly weaker than toughened and the kitchen is not really a place to be taking short-cuts with glass, especially if it comes right down to the benchtop. Also, if the splashback has any notches, powerpoints or other cutouts in it, your glazier will insist it is toughened. Hope this helps :)