My question is - should i sand off or further remove the thinset in order to properly install a new backsplash, or is the old dried thinset still OK to leave on there without issue?
I installed subway tile with an accent strip around the top in a bathroom, and the drywall was severely damaged from removing the previous tile. I ended up doing a skim coat of drywall mud (there was not any thin set mortar left, just chunks and gouges out of the drywall from taking off the old tile with paper torn off everywhere) then painting it to seal the mud and finally sanding it a little to give the thinset a dull surface to adhere to instead of a glossy surface.
If you do try to scrape off the thin set, a bent angle painters' scraper that is used to remove wallpaper works great. If you are going to try a skim coat with mortar, you probably will have to bring the outlets out a little to compensate for the added thickness. Originally I was going to use glass tiles for the accent pieces on my wall, but the contractor I hired to set the tub told me that they are terrible to cut and they show wall imperfections behind them, so I don’t have any experience with working with those.