I think if you don't plan to complete the postbac, then get out ASAP. It will probably hurt you at that particular DO school...doubt that other DO schools would necessarily find out that you enrolled there, even if you decided to apply to them later...
Since you are from California, how about trying to get into Western U., panapply to all your state MD schools (on the small chance you might get in), and then apply to 3-4 Caribbean schools at the same time. I don't think you should agree to do extra semesters (such as MERP program) because it doesn't seem like it would benefit you. You'll just end up paying them more money...