The ceramic needs to be built out. In the following pic you'll see on either side of the window, where the accent stripe comes into the outlets and switches. That stripe, without doing anything to the tumbled marble, would've stood proud by about a shy 1/8", in which case, as you're finding out now, there'd be a gap behind the face plate. You might want to pull the ceramic pieces and build them out a bit.

Joseph Corlett has tons of advice. we use Mapei products. They carry stone cleaners and sealers for different types of stone material.

The top end choice would be to use a beautifully figured piece of alexandrite slab as the backsplash and focal point. It depends on your layout and sight lines. Whether the alexandrite is polished or honed and lighting come into play. Tile choices require taking your component samples to some tile stores and seeing what you like. Patterns like subway, herringbone, stacked, chevron, etc. depend on your interior decor style. Get samples to compare in your home lighting. And compare grout. Urethane and epoxy grout resist staining for lighter colors. Post pictures of the options for feedback.

I would NOT keep those maple cabs as-is w/that granite. You've already chosen the stone and it does not work w/the warm, yellow/orange cabinet stain. **Make sure you're using bulbs in the 4000K temp range otherwise you're going to get warm, yellowish tones. I can see it in your photo** https://www.kylieminteriors.ca/how-light-bulbs-affect-paint-colours-kelvins/ Here, look how much better the stone looks when paired w/actual colors that enhance it. This succulent shade is gorgeous. I'd think about doin the walls, the doors or the kick panel in that shade. It's not a sin to paint them! heck, I painted mine 4 yrs ago and couldn't be happier I actually refaced my doors/drawers because I wanted everything to match. the replacements were solid maple w/MDF center panel. Who is painting for you? make sure you research exactly what their steps are. do they paint off-site? what do they use? spray? what kind of primer? do the sand before AND after the primer? All of these will make a diff in whether or not the paint chips. mine hasn't even after all this time. I would go w/a nice bright white to compliment the stone. you don't want a warm white or a white w/yellow or red undertones. Some good whites by BM is Oxford White or chantilly lace. (do NOT use White Dove. very dingy gray) read these articles on white paint w/your cabinets and trim. very informative. (she also has other links on which is better depending on your direction of light.) https://www.kylieminteriors.ca/4-steps-pick-the-right-white-for-your-trim-cabinets-or-walls/ https://www.kylieminteriors.ca/the-8-best-benjamin-moore-white-paint-colours-undertones-and-more/ another color combo. Eider White is a white that has gray under tones. and SW SeaSalt is a gray/green Two other gray greens that you could think about doing w/that stone. either the door, the trim, the lower cabinets, the front kick panel, etc. BM Rainy Afternoon BM Caldwell Green. This would also look like it could work. you'd have to try a sample. She used that same repose gray on these cabinets. (love the brass handles). If the color looks like this in your kitchen, it would be a perfect shade for your quartzite. Will they be using a lacquer conversion paint? please wait on backsplash tile. no rush. (but a nice gray green tile would look nice. please, no white subway tile. this one is the Newport tile w/either a soft gold pull or the nickel or chrome make sure they fill all the current holes w/a good wood epoxy prior to priming. you want to use diff hardware You sure that stone is Aqua Bella and not Sea Pearl? https://www.houzz.com/discussions/5046402/backsplash-help-for-sea-pearl-quartzite