You've picked a great place to celebrate those milestones and we'll try to answer your questions.
1. Night time attire is typically casual unless you go to the Lychee or Otaheite restaurants where the dress is resort elegant and cocktail elegant respectively.
2. Let them know at check-in that it's your anniversary (congratulations by the way) and that you're wife is celebrating her birthday and they'll do something special.
3. Absolutely! The candlelight dinner is an awesome way to celebrate her birthday and is well worth it. We celebrated our anniversary in the treehouse and love it!
4. Can't help you here but know you'll have others who can post and will.