the platinum reward is free if u get platinum status by June 30th of every year it takes 600 coin's to get this status. And it hasnt been reviled yet what this years reward is.
No, not with extra coins, but you need to have accumulated 600 coins within a Club Nintendo year July 1 - June 30. You can spend the coins on other things and they will still go toward Platinum status.
Nintendo will reveal the Platinum Reward right after the Club Nintendo year. (after July 1st)