No-one will ever see that spot, unless they are deep cleaning or similar, so I don’t think it matters. If that tile has a quarter round piece that fits, you could use it both under the cabinet and on the rest of the backsplash. Nice looking tile work.

I suggest that you run the tile down a row or two behind the stove. Right now you've got a stove with a riser, but down the line you may end up with a stove without one and if you don't have tile down the wall a bit you may end up with a gap there where the edge of the range abuts the tile. Just do it now and be done with it.

No one is ever going to notice the gap you pointed out unless they're sitting off at a distance at a just-so angle where they can see under the cabinets, so I wouldn't worry about it.