In my view it's premature to talk about a postbac or SMP prior to finishing your first go at undergrad. If you can fix things before you graduate, I think that's quite preferable: easier to explain and probably cheaper. In particular, I think a reasonable strategy for you to turn things around is to change your major (to any hard science), retake some classes, take a lot of upper div science, thus add a 5th or 6th year, and show a sharp upward trend after drawing this line in the sand.
But to answer your questions:
1. SMP programs require that you take the MCAT, and score in the mid-20's. If your grades were weak in the basic sciences, such a score might be out of your current reach. Also, EVMS is the only SMP I know of that takes a sub-3.0 GPA. An SMP would likely be the fastest route to med school for you, but quite frankly, it's the most likely route to failure as well. I mean no disrespect; SMP students are in "do or die" mode and you have to be top of the heap to benefit from an SMP.
2. You'll have a hard time finding a structured postbac program that admits people who've already taken the prereqs.
Hope this helps...