We are trying to save money and replaced an old laminate countertop with a newer preform laminate countertop, where the backsplash is part of the laminate countertop. The countertop is an "L" shape, so we have the pre-cut miter ends that join together in the corner. The problem is that the walls seem to have a serious bow to them and where the countertop pieces join in the corner, there is about a 1 inch gap between the backsplash and the wall. The counters are flush up against the wall at the furthest ends from the corner. Is there any way to fill in this gap?
If the walls aren't square, I bet the cabinets aren't square as well. And nothing says your counter top has to be square as well. The best way to hide out of square rooms is to try and match the existing angles. In this case you will need to make the miter cut a little bit off 45, probably 44. A small angle change can make a huge difference when getting to get things to fit. And even then a slight difference in the overhang on the cabinets will not be as noticeable as any means trying to fix the gap between the counter and wall.
This rough-cut stone backsplash is pretty popular these days and is super easy to install (just did our kitchen with a similar product). It can be bonded right to the sheetrock. It should be thick enough to hide your gap. If you need it a bit thicker, you can screw some backer-board to the wall and bond to that. If it's your style, go for it.
You need to plane your bench top in to fit. Then you will be able to push it back.
Then check if you have the same over hang from cupboards to bench top. If it's a little bit out don't worry about it as long as it looks ok. (can't get perfect with old kitchen)
Then if there is still a small gap you need to fill it with no more gaps / silicone. Need to silicone it anyway to stop water damage.