I would book Delta to GCM, and UA to Hawaii. I think UA has better options into Hawaii. I never thought UA had a lot of options to the Caribbean, maybe they do into GCM, I don't know. I'd bet DL has better options, I just think of them as more of a southern airline. As far as the return leg, I'd call DL and see what their policy is. You may be able to book the return even if it's too many days out. You could always call and agent and reserve/hold the awards tix with a bogus return (next day even). Then continue to extend the reservation until the return you want opens up/becomes available. I've done this with UA and CO. I just kept calling, everyday, but always before my reservation expired and asked the agent to re-reserve the trip, therefore extending the time before the hold expires. You can do this a couple of times. If the agent won't, then politely excuse yourself, hang up and try a different agent. You'll find someone willing to work with you. If the return opens up, great you're in luck. I'd guess that you'll be fine, you're planning way ahead.

Good luck, Bubba