So if I understand this correctly -- good performance in a SMP will not overcome a low undergrad or postbacc gpa?
No, a good performance in an SMP DOES compensate for low undergrad/postbac GPAs. The problem is that a 2.5 is too low to get admitted to an SMP anyone has ever heard of with the possible exception of the DO SMPs (that you need to look into). You need to get your GPA up to at least a 2.75 before any allo SMP will look at you, and being below a 3.0 an admission is still gonna be rough anywhere but VCU.
The second problem, as Instawaiter mentioned, is that a lot of medical schools autoscreen out anyone with less than a 3.0. If you don't get your GPA to a 3.0 you'll either need to eliminate those schools from application list or personally call their ADCOMs to see if you can get them to un-autoscreen you based on your SMP grades. I tried the phone call thing to get by VCU's autoscreen, BTW. They weren't sympathetic. An SMP can't help you if no human being ever sees your app.
Honestly, if it would take 2 full years of aditional undergrad just to get you to a 3.0, the Caribbean is probably something you should seriously consider. With that kind of MCAT you could get into a top 4 school as early as this coming spring. Now lots of students who start the islands schools either don't graduate or don't get a residency (leaving you smothered under debt forever), so make sure you're absolutely SURE you've fixed the problem that got you the 2.5 in the first place. However, if you CAN rock your classes and the boards the islands might get you a medical degree almost half a decade before you could reasonably excpect to graduate from a US school. Also, in your medical career they're only going to care about the last thing you graduated from, so if you finish Caribbean school followed by a US residency you're not going to be treated as a Carib graduate, but rather a US residency diplomate just like everyone else.
So to sum up, I'd say either
1) DO SMP
or
2) Caribbean Big 4 school.
How much of a factor does cumulative gpa factor into ones attractiveness as an applicant as opposed to just recent coursework? Would it be enough to get 30 credits of straight A's (something I might be able to do in one year if I can cut my work hrs.) followed by either an SMP (if they'll accept me) or applying directly after this to DO schools?
Trends are nice but cumulative GPA seems to matter most, based on what people have reported here. Also, no offense, you're probably not going to get straight As. Very few people can manage a 4.0 and the ones that can generally never get anything else. If you make a truely stellar turnaround you might make a 3.6 or maybe even a 3.7. You should probably base your calculations on realistic numbers. As for making this turnaround while taking a full courseload and working full time or near full time... seriously? Again, be realistic. If you got a 2.5 as a full time student how could you get a 4.0 as a full time worker with a full student schedule? If you are going to take courses try not to go nuts on the number of credit hours. Every C you get digs the hole a little deeper.
One more question -- is it generally recommended to retake core prereq courses that one has done badly in or move on to other courses in the same field but mid and/or upper level? I didn't do very well in chemistry and was thinking of taking additional but different chem classes to try to make up for this.
You need to retake anything you got less than a C in, don't retake anything you got a B in. For Cs you can either retake them or not at your discression but only retake them if you're SURE you can get As from here on out. Also obviously retakes are a better idea if you're planning on going DO, since they only take your most recent grade in a course.