Hi, I would suggest a pale grey grout as it is subtle and much more forgiving than white. I have attached a couple of examples where I have used a mix of dark and light tiles with a light grey grout. Hope this helps, David.

A local glass company should be able to cut a small piece for you. I wouldn't advice though putting it over an existing tile as water/condensation could get inbetween. We normally use a piece or solid worktop material at the base of a recess but a tile should still be absolutely fine as long as it is fitted correctly.

I’d say white for the walls but definitely grey for the floor. Contrast can be achieved with towels and green leafy plants.

Choose the one closest to the tile colour. No 1 or 2 I can't really tell from pics. 3 is a non starter unless you are deliberately looking for a grid effect. I think your tiles are grey rectified porcelain? If this is the case you can go as small as 2mm with your grout. This will make cleaning much easier. I had them once and they were fantastic. So easy to keep clean and very crisp looking.