I don't know how you value your miles, but paying 120K to the Caribbean you'd almost always be better off paying cash. As for your other questions:

1. I personally think the rates will go down. It's not at all uncommon to see 10K each way to AXA. You could also consider SXM. A short ferry connects the two islands.
2. You've verified, but I'll reiterate, no problem getting points refunded on any cancellation up to departure.
3. Unlikely. Your 1 ticket is immaterial to how full the entire aircraft is. I find the best deals to the Caribbean come at the last minute.