My counter seam with a fine-grained quartz is a hair's width. Part of the reason is that they used a machine that pulls the slabs together. Not sure that's possible with a backsplash. Are you doing a full quartz backsplash? Some people mistakenly do 4" of the counter material and a tiled backsplash above that. You'd want to avoid that.

I had a kitchen countertop with the built in 4" backsplash for almost 20 years. The wall above the small splash was painted in a washable paint. I never had any problems with it. That is a perfectly acceptable way to go. I recently changed out counters to a new quartz and did do a simple subway backsplash. I like it very much and it was not too expensive. The tile was about $230 and I installed it myself. My neighbor was charges $400 for her install of the same tile, but that is low for a backsplash install.

I don't have a suggestion for sealant - but in addition to ideas you get here, make sure to read up on what the manufacturer and your supplier suggest.

This site is awesome. Timely conversation, as our designer recommended we move our range from a peninsula (that will become an island) to an outside wall, for better function. It is a GE Cafe dual fuel 30 inch range with the island trim, put in just over 3 years ago. After finding this thread, found an online install information, with the only recommendation to be 2 1/2 inches from the wall in the back and 3 inches clearance on the sides, and they had no recommendations for the wall behind the stove otherwise. I called GE and the service department there confirmed that this island trim oven was ok to use on a wall counter, and that to just follow code for the wall behind it. No recommendations other than the 2 1/2 inches from the back. I discussed this with our designer, who had just recommended only 24 inch counters, and the quartz backsplash, who was pretty noncommital about needing any changes. When I did say I cook sourdough bread at least twice a week, with 500 degree ovens for warmup, she said to get tile or real stone, but didn't need to change the counter depth as the tile was the fireproofing. It will be fairly easy for us to deepen the counter about 3 inches, I will just need to have a 30 inch sink rather than a 33 inch sink on the counter that will now be 3 inches less wide. But that will be just me and our countertop/cabinet group, not sure if it is worth it to convince our designer of what is common sense to me. Now we get to work on backsplashes. This range has the simmer and smaller burners in the back, with the larger speed boil burners in the front, so not sure what the fireproof wall in the back needs to be.