shirt and tie? that might just be considered "very dressy" for Palo's based on what I saw. there I was in a suit/coat/tie for dinner, and at the table next to us are some folks in casual dress as if they were just going to the main dining - simple button down shirt with open neck. Oh, don't let me forget to mention one of them was wearing shoes with no socks!
nobody will say anything if he's wearing a tie with no coat.
as far as going to Palo and the dining rooms - since you have late seating, you could easily schedule Palo dinner at 6PM and still go to your regular seating afterwards. you might be extremely stuffed in doing so, but you could still go to the main dining and just have an appetizer/dessert.
we took our first cruise on the Wonder as a 4 day and didn't do Palo. This past trip, we had dinner at Palo on the night of our 10th anniversary. Let me say, that in comparison, it is no big deal to give up one night of your regular dinner seating to have Palo instead - it is well worth it. though the main restaurants have unique themes, the atmosphere is not all that different between them - same servers, same folks at your table, same noisy environment with lots of people and kids all around. at Palo, it is intimate, quiet, a wonderful ocean view, you are not rushed, the servers are a cut above those in the main restaurants and don't have to handle as many people, and the food is simply fabulous and cooked to order. add a nice bottle of wine and you've got a very memorable evening from your cruise. Let me say again, you do not want to miss a dinner at Palo if you have the opportunity to do it.