It's "one of those things". Glass typically does not need a sealer. But glass tiles with decorative finishes can actually have slight fissures or a texture to that decorative finish, and that surface finish can benefit from a sealer. Somewhat the same with slate. A true high-quality HARD slate is a thing to behold. The waterline tile in my swimming pool is slate. It's survived well over a decade of winter freeze/thaw cycles with no water absorption and without the resulting spalling or pitting that would have ruined a more porous stone. However... "Slate" can be many things these days. A honed slate will usually originate from a better quality and harder slate than a some cleft slate because some softer slates are too soft to survive the honing process. They'd disintegrate in the process, so they get marketed as cleft. That's not an indictment of all cleft slate. Just the soft flaky ones. If the manufacturer says to seal your tile, then seal it. As to how often it needs to be resealed, you'll pretty much get a generic "reseal it whenever is needs resealing" answer. When you first seal the slate, you may see water beading on the slate. Over time the water may eventually just create wet marks on the slate instead of beading up, or the wall tile may appear wet several hours and after a shower where it dried faster in earlier days. That might be a good time to let the shower fully dry and then reseal. A good quality impregnating sealer should perform well for years. Two years, three years, five years even. It's the cleaning methods that can mess things up. If you use shower products (soaps, lotions and potions) that gunk up your tile and grout with lotion/potion residue, you'll have to clean and scrub more often. When you do have to clean, if you use a cleaner that is NOT pH neutral, one that is more harsh on the grout and tile and sealer? Then you'll have to seal more often. If you seal with a good impregnating sealer... If you don't use harsh cleaning products... Then you might only have to reseal every 3-5 years. I'd ask W-Z what sealer they recommend. I'd follow the guidelines of that sealer. So overall, a somewhat ambiguous answer. "It depends...". Report back if you get any added info or of you have any other Q's.

Either a single or double row. I just finished installing a backsplash about an hour ago that had the same question (field was 6x6 diagonal, but same question about the accent, only these tiles were 1/2"x1/2")). We ended up doing something a little different. My stepson's going back to grout tomorrow, so I'll see if he can get pics for me. We ended up doing two rows, then a piece of field tile about 1 1/4", and then another two rows, and then back to the diagonal field. It made for a really nice accent band, just a bit higher than the middle of the backsplash.

That looks like a lovely install! It is so nice to see how they look when just laid right next to each other without spacers. And, it is probably a much easier install process too. Thanks much!

@AJCN -- thanks for the tip on the Frost grout! Will definitely consider. @GreenDesigns - someone woke up on the wrong side of the bed this morning, eh? Take it easy there buddy. We are not home interior design "pros". We sell decorative table linens that we help design in India and import to USA. I was posting regarding a one off request from someone regarding something which we typically don't do asking us for some feedback since we are not interior designers. Thus, I posted on Houzz asking for experience and respectful feedback from professionals (something you lack) that are IN the home design space (as we are NOT). Thanks for the education on the Internet... really needed that. Instead of critizing others, you should re-evaluate your own Houzz profile and your web presence. Your logo isn't very appealing, to say the least...