How to fill large 1" gap between preform laminate countertop and wall
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.