No you cant book a cruise that is not currently open to be booked.

YOu book a cruise that is as far in the future as you can but know that you will eventually cancel that cruise and then transfer the onboard booking to the cruise you eventually take. Even the cruise consultants on board advise you that is the way to handle this now.

At this time there is no dollar maximum on the 5%. They have a new "wrinkle" in the wording of the onboard bookings though that say the discount will be 0%, 5% or even a higher percentage on certain cruises. To date I have seen nothing but 5% but that clause is in there if they wish to not allow the onboard booking on a specific cruise or if they wish to enhance a date but adding more discount for an onboard booking.