The Mexican cruises are not completely sold out. Some of the categories for specific dates might be sold out. I believe there is very limited (if any) availability for the 14 day cruises at this point.
If you have some dates in mind you can check out the disneycruise line site and just get an idea of what is available or there are many travel agencies that are promoted on this site.
For now, the Mexican cruises are touted as a special attraction for Disney's 50th anniversary. There are some rumors that Disney is checking out the possible market of having one ship positioned out there (there are also rumors that a 3rd ship is already under construction) but at this time nothing is final.
I don't know much about the DVC point thing. Might depend on what the availability is? Maybe someone else can help you more with that.
Brenda