Assuming you're installing cabinetry along the entire wall, I'd do this:
Find the point on the back wall that sticks out the most.
Measure out one cabinet depth and mark a line over the entire length of the floor. (Use a grout line as a reference-- even if your tiles are off-square, these are the lines your eyes will pick up as square.)
Install cabinets to ...
If you put the cabinet tight to the right wall and then install the fridge next to it the cabinet will look out of square as it will be 90 degrees from the right wall. A gap will form between the right cabinet and the fridge or the fridge will be skewed.
So as the cabinets run looking at your picture from right wall to left the gap in the back would have ...
I'd push the cab against the back wall and scribe a filler strip (somewhere between 1/2" and 1-1/2") on the right side.
But realistically, if the counter edge on the left (beside the fridge) is perpendicular to the wall, nobody but you would ever notice.
Acetone is great for dissolving glue and adhesive. Try pouring some along the top of the backsplash and let it soak in between the wall and backsplash. After about 15 minutes try to pry it gently with a mud knife, looking for a little loosening of the backsplash. Apply more acetone as needed. Good luck.