I had similar questions and was going to start a separate thread. Long story short: My parents have a timeshare (with exchange privileges with Interval International). They have one week they won't be able to use, so they gave it to my wife and I. Naturally, my first thought is, "Can I use this timeshare in Negril?" Well, it turns out that there are 2 places in Negril that are affiliated with Interval International - Azul Sensatori and Sunset at the Palms. Both of them require that you purchase their AI plan for $100+ in addition to the timeshare cost. Also, it appears that availability is limited. Basically, these resorts offer up their unsold hotel rooms to Interval, just to fill the space. so, If the hotel is fully booked during high season, there are no timeshares available.
The cost of the AI plan is a deal breaker for me. I'm not going to pay $150/ day just to go out and eat at different restaurants each night. For $150, I can pretty much stay at any hotel of my choice. I thought I read that the Beachcomber Club had timeshares available, but they don't appear to be in the Interval directory. Instead, I am looking at the Jamaica Inn, in Ocho Rios. It's not Negril, but it is regarded as one of the best hotels on the island.
If anybody has any experience with timeshares in Negril or Jamaica, I'd be interested in hearing your thoughts.