Thanks for the info, just wanted to confirm so all know.

1) It adds to the same account, make sure you add it under: Mog Station > Account Services > Add Windows Service (Or ADD PS3 depending), Enter your Game Serial (not the Square Enix community Serial), and you're done.

2) You do get the extra 30-days added, yay!

3) Only one active-subscription is needed to play on either PS3 or PC, however both cannot be played at the same time. Only one or the other.