I'll let others reply to dress code questions.
As for making dinner reservations, you can wait until onboard to make dinner reservations at La Terrazza and The Grill but be aware that you will be competing for spots that others have made prior to boarding. You may not be able to eat at the time you prefer. I just debarked from the Dawn, and while waiting for the Maitre'd at La Terrazza, a party of 3, young man and older couple (grandparents?) asked for a table, and was told it was sold out, and return at 8:30 to see if there were cancellations or no-shows. He claimed not to know reservations were necessary. So, if disappointment isn't a problem, you can wait. Silver Note is especially difficult to get a reservation for. Don't know if Whisper has that venue.
Don't you have to book ahead for Regent specialty restaurants?