The latest ressie for Palo's on the first night on the Magic at least was either 8 or 8:30. That is their slowest night so I guess they pack up a little bit early.
On every cruise I've been on, Palo has been open from 6:00pm to 10:00pm on sea days, and 6-9:00 on port days. I don't know if that means you can make reservations for 10:00pm, but that is what the hours are listed as in the Navigator. Based on that, it seems that you should certainly be able to make 9:00pm reservations on a sea day.
Note that all of my DCL cruises have been on the Magic, so your mileage may vary on the Wonder.
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