can you take advantage of price reductions after final payment? Only if the final payment date has not arrived and passed. Some people make final payment before it is due. The cruise lines don't care, if the date has not arrived they will adjust, if it has the best they will do is an upgrade. Of course if you have a penthouse suite there isn't many places to upgrade you to. Your question about the beds, my guess is they would be fine, but hopefully you can get a better answer from someone else. Are you using a travel agent or directly through NCL? Either way they can help you, especially if you call NCL.
By the way, welcome to cruisemates!!!