We tiled down behind the range, lower than the recommended 6" below the range. No fire code issues there. Also, although the trim is sold as "island" trim, it is perfectly safe to use it against a wall (according to Wolf, and our appliance store--we did not order the trim against any objections for our intended use). It was only when we had the burner issues that some of the svce techs pointed to the trim as a contributor to this problem. Not all techs agreed with that "theory" though, and there were also a number of people with taller trims on their Wolf AG's who were experiencing the same problem as well. I am sure having the lower trim does contribute to the backsplash discoloration/grime. No question about that. However, we 1) much prefer the look of more tile/less range trim, and 2) like having full use of/access to our rear burners. Higher trim can, and often does permanently discolor. Sticking a piece of foil behind the trim when roasting or using high heat for extended periods is simple enough to do, and I can toss it out when done vs scrubbing the backsplash each time I cook certain foods.

At this point there is no way to level the tiles short of tearing them out. The installer could have made the accent strip level with the 2 x 8s by putting a deeper bed of thinset under it. In a perfect job, he would not have that edge of tile slightly out of alignment. Having said that, if this photo is representative of the whole job, I wouldn't consider it so bad I'd want to start over. In fact, I kind of think it adds to travertine's charm that every tile placement isn't just perfect. In a little while you won't even notice these imperfections and visitors to your kitchen are unlikely to ever notice. We installed our travertine ourselves. The blue glass tiles were a separate sheet that we cut up to replace the odd travertine tile with. Like your accent strip, the blue glass tiles are not as thick as the travertine sheets. We noticed when we were wiping away the excess grout that we didn't get a couple of them just so, but really, this is the first time I've thought of it since the first week it was installed and I'd have to go on a hunt to find the protruders.

We sealed ours when it was installed. We used water-based Aqua Gold(?) sealer; sold to us by Artistic Tile when we got our backsplash tile from them. It's what they recommended, so we wanted to use their preferred sealer. We used a solvent based sealer elsewhere (bathrooms, FR floor...). Probably could have used that on our backsplash as well, but didn't get/use it until a few years after the kitchen reno.

I like it, but I'm biased - I used a marble backsplash with my greige kitchen cabinets. Mine is a chevron pattern, and we used a light grout which doesn't make each individual tile stand out as much as yours do. You might try changing the grout to see if that helps. But if what you really want is a white subway look, it's not so hard to change out a backsplash - if you redo, tiling up to the ceiling behind the range top might be nice. Fwiw I doubt this will be a make-or-break issue for future buyers - your kitchen is lovely, and anyone who is walks away from a sale over a marble backsplash probably wasn't a serious prospect anyway!