Why wasting time running around for mining when you can get it right from the mart with 100% rate. Adding the rest of the mega stones for mons like lopunny, absol, gengar, aggron, bannette, etc. would be better
The chance of getting a shiny starter when you first join the game is the same with hunting one in the wild, so it's mostly based on luck, as for getting a shiny on lv10 something, I don't think that's possible.
If it's the starter pokemon you chose at the beginning of the game then you can't trade it, but those you caught in the wild can be traded. If that greninja is caught from wild but can't trade then maybe it's bugged, try reload the game or report the bug to dev staff if the bug continues.