I always use color matched grout caulk at the interface between my backsplash and counters. Grout gets gunky and cracks down there. I grout and then I dig out all the grout that is in the area between my tile and my counters while the grout is wet, then I caulk the next day. The Tile Shop did not have the grout caulk that matched my grout on my last backsplash but I was able to find it and order it online. It is never a perfect match (one is a cement product and one is a adhesive product so it is hard to get the colors perfect) but it is pretty close and looks good. Occasionally I will choose my grout caulk in a color that either matches the tile or counter top material instead of the grout. You would still need to add something to your Schluter edge (clear silicone most likely) because you need to create a waterproof seal to stop water/liquids from wicking from the countertop behind your tile to your drywall and down to your cabinetry.

https://tile-assn.site-ym.com/store/ViewProduct.aspx?id=3484938 The link I posted above which is the same link from 10 post's ago for the TCNA Handbook is what you need.(Tile Council of North America) If you are intending on walking your GC through this instead of getting a professional all the responsibility falls on your shoulders. Your GC is not competent enough to be taking on this project. If he was he would have researched the proper methods prior but instead he argues with you. You are making the choice now to be the GC. I think you are making a big mistake but it's your shower. Best of luck and I hope it all works out for you.

ebennan, Im sorry you had this dilemma! I'm currently in the process of having a schluter installed around our niche and was told that even at 45 degree cuts there might be a gap and should consider using schluter edges and joints that are installed between the two schluters. Those parts are custom order and would delay our bathroom completion. Did you end up tearing out or grouting yours? How did it turn out?

You can use a Dilex and skip the caulk. Any other edge choice needs to still be caulked, so theres no point in using them. Dilex isn't as readily available as a Rondec or Jolly, so it may come down to availability.