We looove CSA!! I will try to tackle some of your questions.... as far as what we do when booking a room, we always book the lowest level room. The atrium rooms (lowest level) are awesome! But, we're also repeaters, so we may or may not get an upgrade when we arrive - it all depends. We do love the atriums so if we get "stuck" there, that's fine by us!
We have always booked our airfare separately. Usually we book our trip so far out that airfare is not an option to book at the time anyhow. Plus, I've been able to find better deals on my own than what is offered through the airfare package.
Bus ride is about an hour and a half. It's not a bad ride, and you'll get to see a lot of the countryside. Hopefully you'll get a driver who wants to tell you about everything you see (sometimes we have, sometimes we haven't) -- but even if you don't, you can ask your driver any questions and they will gladly answer. (Also, just a side note, you tip your driver, and also your luggage handlers at the airport. The transfer is included, but tips are appreciated.)
I don't know anything about doing a vow renewal but I know they offer it. I think it's on the page about weddings.
If it rains, typically, you will get an afternoon shower for an hour or so and then it clears up. You're not going during hurricane season so that minimizes your risk of rainy days. I haven't been in April but I would think that's a great time of year to visit (I've been in March and also in June). Even if it DOES rain, the shower will give you an opportunity to take a nap or just chill for a bit. If you look at the forecast for Jamaica, it's going to say "rain every day". It's because it's in the tropics and they get showers that typically don't last very long almost every single day.
Good luck with your planning, and happy vow renewal (and anniversary)!!
Melea & Joseph
CN - '06, '07, '08
CSS and CSA - '09
CSA - '10