Formatting will delete the account from your Wii U, it just doesn't unlink their NNID, they'd just need to call Nintendo if they ever get another Wii U, so it's their problem.
Oh, okay! That makes more sense. I can dump GBA, DS, 3DS, & Wii ROMs. For the other games I own, is it legal to use someone else's backup, or do I need to find a way to dump those?
Yes, but you'd need to have the files for the Japanese language for FFX, not Puyo Puyo Tetris, because this guide is (basically) how to replace the Puyo Puyo Tetris voice files with Japanese voices. And the folder in /atmosphere/titles would have a different name to indicate FFX instead of Puyo...