QuickFix; Short answers to tricky questions.
A You can, of course, paint over them with the same colour paint as that used on the chimney breast. For a more professional finish, put the surround in place and draw round the brackets with a pencil, then remove the surround. Chisel out the plaster slightly where the brackets will sit, then they'll be sunk into the wall when back in place. Fill over the top with a suitable filler, sand, wipe clean and paint - and you'll never know they were there.