how to fix an (extremely?) uneven wall before tiling
I would take out/off the drywall if not from the whole wall then from the cabinets up, take off the window trim , casing and sill. Then address any unevenness of the studs with furring strips or shims.  Since the wall is open I would replace all the electrical boxes with the new adjustable depth boxes so you can set them to match your tiled wall. I would then use Denshield in place of drywall, it is similar to drywall, cuts and installs the same, 1/2 thick with a coating on it specifically made for tile. You would only use it where tile would be installed so if the tile will be going all the way to the corner then install it to the corner. If not install it so that it will be just inside ( an inch ) of the tile line so that the texture of the denshield is not visible after tile is installed.  install new casing and sill on the window, straight and true so tiling will be less complicated. if you are wrapping the tile into the window you can use the denshield to case and or sill.