Here is a link to google, how that backsplash should have been installed. There are some u tube videos too. I watched one video, where they used grout, between the countertop and the backsplash, and grout should not be used there. Caulking shud be done between backsplash and countertop. My backsplash is also caulked in all the corners, rather than grouted. Once you watch these videos and read these articles, you will see that your thinset was put on improperly, and thats why your tiles are falling off. Please read carefully. https://www.google.com/search?source=hp&ei=uI4VW7HdLszKjwTtrqSoAw&q=how+to+install+glass+subway+tile+backsplash&oq=How+to+install+glass+subway+tile+&gs_l=psy-ab.1.0.0l3j0i22i30k1l3.3755.12143.0.14244.41.29.3.9.9.0.173.3348.9j20.29.0....0...1c.1.64.psy-ab..0.41.3556...0i131k1j33i22i29i30k1.0.70wb9L5GlO8

Get a sample of your cabinet doors, and then bring it to visit the samples of counters in your favorite show room. Nobody can guess.

If I understand correctly, the wall cabinets on the sink wall will be replaced. Give that, I would run the tile to the ceiling, like the range wall

Thanks everyone for your responses. I‘ve just gotten a few more photos sent to me. This time from more of a distance. They are duplicates. One with the overhead light in the kitchen tuned on = light above sink, under cabinet and ones without that light on. From what he was telling me, they tend to use that particular light in their kitchen more than the light from the proper light switch. I think that may be enhancing the “flaws” that they see. @joseph_corlett do you advise that they contact the installer to fill the hole, or should/can they do this themselves? I think its been about 16 days from initial install. @Sammie J Just got photos taken a bit further away. @Kelly Zamonski @User They were under the impression that a tile backsplash would further accentuate the uneven wall on that side. Luckily none of us noticed it until they moved the large item they have in that corner. The choice of GC is a bit iffy. Thats what I told them. Do you recommend simply leaving it alone at this point or finding another GC? It may just cause more time, money, and stress. I’m of the school of thought that it is what it is at this point. No further aggravation is needed for them Especially because of the potential cost. Like I said, most of these photos have the under cabinet light on and then off to show the contrast. They mainly use the under cabinet light which highlights the issues they see - well, we all saw. We all know nothing about remodels/updates or what to expect from them.