Yes you can buy the drink packages once you board. As soon as you board there will be tables from all the main departments advertising things like drink packages, spa treatments, kids clubs (not that you need that one), etc. I think you can also buy them from the bars but don't quote me on that one as I don't use the drink packages myself.

As to the wine, if you want to bring two bottles onboard it needs to be in your carryon when you board the ship, not your checked baggage. For flying, I always wrap in bubble wrap, put in a large ziploc bag, and then put in the middle of a pile of clothes. No issues ever (this is for coming home, but it will work the same way going to the ship). When you get to FLL, make sure to take the wine out of your checked bag and put in your carryon.

I think the ATM charges are something like $6, and the machines can run out of cash/not be working. You can also get cash from the cashier in the casino - I think the percentage there is 3%. And I believe you can cash checks at guest relations. I'd say $1,000 cash should be more than enough for a short cruise. If you need more, I think the fees for the ATM or the Casino are worth it rather than carrying large amounts of cash.

Sorry I haven't been to CocoCay since they added the cabanas so no advice there.