You will have to go back to your watchlist and remember what the price was previously. This assumes you are doing this on the same device. If not logged in and you look at the watchlist on another device, it will be blank. Using a site like cruiseplum, which has about a year of price tracking history, might be a better choice.
Yes (the app will list the drinks), but usually you don't need to go that detailed - as long you drank on average at least 5 alcoholic drinks per day, you are likely ahead, financially.
Most of the time they are talking about a single category (since you can move within a category at no charge). It's actually fairly rare for an entire sailing to be sold out till close to sailing. RCI likes playing games where they'll pull an entire sailing for "inventory management" (and people think it's sold out) but then the sailing comes right back.