Its all about registering that PSN code and Product Key to that specific square enix account.

You have to buy a PS3 version of the game to get those codes sadly. But after you open the case and get the codes and attach them to your account , only one copy of the game is needed to play.

As for the sqaure enix launch Page greying out the ID, that is attached to your PS3 local account then when you sign in with your psn account for the first it locks that id to it. Not sure why but it does. Hope that helps