My D is at MMI right now with approximately 15 Coast Guard Prep cadets. When we visited the CGA, they said that they only contracted for 15 at MMI. The other 45 went to NMMI. I'm not sure how the kids were matched with prep schools, but they didn't really have any say in the decision (at least my D didn't). When we asked, they just said that they took the needs of each cadet into consideration and each school had their particular strengths and they tried to match cadets to the best school for them. In one case, a MMI prep school cadet is a NMMI high school graduate so I'm guessing they wanted to take him out of his comfort zone.
As far as when they will receive their appointments: My D says that her Academy advisor told them that they might hear as early as February, as late as April. We know they hear later than the sponsored cadets for other academies (10 West Pointers heard last week and another expected their appointments this week).