With regards to the place holder, since that expires 2 years after you booked it, I have been booking that on the last day of the cruise. However, on our last cruise, we were speaking with the CM on the penultimate day and she offered to take the form then but charge it on the last day, thus avoiding us bothering them on the last day.If you know you'd like to book a future cruise or a place holder, I'd recommend going to the booking desk early in your cruise. You will be able to do it quickly whereas if you wait until later in your cruise the lines will just keep getting longer and longer.
So that may also be worth bearing in mind, if you want to eek out an extra few days on your place holder. Also, you can just drop that form off in their box on the morning of the last full day of the cruise and they'll process it that day anyway without you needing to queue.