To fill in some details. I'm a magician and a member of the Magic Castle in Hollywood. The Castle is a private club that requires an invitation from a member for entry. You are correct, guests who are not accompanied by a member must have a dinner reservation and purchase at least one entree.
From what I've read, the drinks on board the Sun Princess bear no relation to the drinks at the Magic Castle (the bars and dining room do have a small mocktail menu). I've been to the 360 restaurant and the Chef's Table on the Discovery Princess and in each case, you can get mocktails instead of alcohol. However, the price for those experiences is not lowered due to the lack of alcoholic drinks. I imagine it will be the same for Spellbound.