If you are going to tile the edges, then you need backer, but it doesn't have to be 1/2" thick. Make sure to use thinset under the cbu and use the appropriate screws to hold it down. Tape the edges, too, with the special mesh tape designed for contact with tile (not the same, but looks like, that used on drywall). 2" should be okay, but it depends on the handles and cabinets you have. Consider the size of tile and the grout line, too. Tweaking it so you minimize cut tiles is a good idea when possible. You don't want any small slivers of tile.
What are you using on the edge?