My understanding is that a proper, effective mirror coating would add so much weight to the missle that it couldn't deliver an effective payload for a usable range.
Maybe Alamo-girl knows the details?
Not sure. The metal outer cover could be merely polished, with no weight increment. And a mirror reflecting only 95% of the falling light [i.e. rather lousy mirror] would be absorbing 16 times less energy than a "neutral grey" object. A better quality mirror would necessitate even higher laser beam power levels.