anaewlo - did all this just over 12 months ago so know how valuable an extra pair or eyes and a clearer head can help. The "little" things are the ones so easily missed and the ones that make you ask "what was I thinking" or "how did I miss that". You could take the tiles up higher immediately behind the hotplates, the range hood would then tie it all together and you could even have some nice artwork each side of it. The only concern I have - are the 2 tiles in the second row centred over your hotplates? Fine if they are as you will have that high part nice and even. If not, I'd a have good think about it. Otherwise, assuming you will paint the walls in a semi gloss paint, the lower tiles will take the brunt of the oil and what goes on the walls will be pretty easy to wash off, it is just a matter of wiping it down after each cooking session. There are times to match things up and there are times to blend. With your downlights, keep to white (assuming you will paint your ceilings white). this way they tend to disappear into the ceiling

Ouch! redoing the job means scraping out the old grout and replacing. In my limited experience tilers hate doing this and may miss bits, I don't think the grout is as solid afterward either, ok on walls not so good on busy floors. If the grout hasn't been sealed and given that it is so absorbent anyway it should be simple to recolour, just test first as it may change colour when it dries. Then use a quality tile and grout sealer, worth the time imho. I would hesitate to tell a professional tiler which brand of grout to use unless you have experience, they differ in quality, colour is up to you of course.

It's a lovely tile! I'd go light-mid grey (basically like the picture!), if you go dark then it's pointless having the interesting hexagonal shape and the veins will look a bit odd stopping and starting, and if you go too light it will compete with the veins in the marble and look to busy.

Agree with Kate. The lighter colour in your bench would be a good colour for a glass splash back. Your bench top is wayyyy too busy, for any tile. Something simple would be my choice. I have included a colour chart and photo of the splash back colour that I think would work.