Payment status has no bearing on ability to book things. Ask others on your sailing in the Roll Call thread if they are having issues with your particular sailing.
Like the pre-cruise planner, on board you may have discounts. If you know for sure you will buy it, buy it pre-cruise and if by rare chance it’s lower on board, cancel (you will get refundable OBC) and buy it on board.
At the bottom of the main page of the app there should be a link to browse ships - that should list all of the ships in the fleet (and all sailings
for about the next three months). She is moving to Cape Liberty this week, so the 5/16 sailing is the first one that will have useful info, though it may not be complete - it should be by Friday.