I have a 4” stone backsplash in the kitchen. We are adding a mural (6x6 tumbled travertine) behind the cooktop surrounded by 4x4 travertine tiles above the backsplash. The designer at Best Tile said grout was optional - I can place them next to each other and not grout given I already have a backsplash. Id rather not grout. Is this acceptable? If I do grout, is it best to use a cake decorating bag and squeeze the grout in between the tiles? A complication - I looked into using a tile adhesive mat to make it easier, but the instructions say to grout to lock the tiles in place. So, I’m confused as to what to do. Using mortar makes a mess (I do anyways). Maybe use the mat n then the bag to grout?

Installed cement board (hardibacker) for a tube surround. I cut the whole for the valve a little to big. I mounted a piece of cement board on the back side. My question is what material can I use to fill I. The cement board ? Mastic/cement? Or should I replace the cement board? Thanks