I agree that at first look it seems like self-preps at MMI (and likely at all prep schools) are an uphill climb. And honestly, it is. The main reason is while the sponsored students just have to show they can pass the courses satisfactorily, self-preps still have to compete against all the other applicants in the nations. As was stated earlier, for example, if you're applying to West Point and you're not near the top of the MOC list, it becomes tougher to get that appointment. While B's and C's get degrees, in the competitive world of SA appointments, you need to be shooting for A's in your college courses.
To me, if a student gets a path to a commission, then we've had a success. Another example, which is not shown on the chart, is how many students apply to multiple academies. Every year I see a student who applied to CGA get accepted to MMA, or vice versa. Two years ago I had a student who was primary Air Force instead accept an appointment to CGA. In addition, every year we have students who decide that they don't want to go to an academy, but instead transfer to a civilian university. Usually it's because the university offers programs that aren't available at the SA's, like the various flight programs at Embry-Riddle.
I guess what I'm trying to say is there is no yellow brick road to an appointment. Wherever you go, whether it is a prep school like NWP or Graystone, an MJC like MMI or NMMI, or a university or college on your own, look for a school that gives you options you can be satisfied with. If you have strong SAT/ACT scores, then a community college or university with a challenging curriculum might be the way to go. If you want the academy lifestyle with close ties to admissions departments, then there's the MJCs. The private prep schools are great in the scheduling and academic drills, as well as ties to the academies, so that may be the way to go if you want to reinforce study habits. In every case you'll get a chance to learn how to learn, develop your academic skills, and come out a more competitive candidate in the end.
Please contact the admissions departments of the schools you are interested in. Give each of them a chance to tell you what they have to offer as well as what makes them unique. Also, look into alternative paths to commissions like ROTC or Officer Candidate School programs. Whichever path you choose, I wish you good luck!