I would ask the Concierge that question. As for confirming the reservation, I wouldn't worry too much about that. I have never had a Concierge not get a reservation done correctly...but it never hurts !
We have had wildly different experiences with BOGO on different ships. Sometimes we are required to book by day 2 and pay up front, sometimes we can book and pay anytime and sometimes we were restricted to using our BOGO only on nights 1 and/or 2 (although, this "restrictive" situation has been by far, in the minority).
On our most recent sailing (Wonder) we were told to just go to the restaurant (I think it was Chops on night #4) and pay when we were done. It's a real shoulder-shrugger as to how they will handle it...but the Concierge will help you with this.