Honestly you're far more likely to get into an in-state college than SFSU's post-bac. An in-state school has more reason to take you (you're a resident) and they have a much higher acceptance rate, even for second bachelors. I don't know SFSU's acceptance rate, but lots of people get turned away. SFSU wants to accept as many people as they can like any post-bac (more income), but like most quality programs out there, they will only accept people they believe will have a strong chance to get into medical school (they want to keep their statistics up, just like any decent post-bac program). If you're really set on SFSU, e-mail their program director and ask for his insight. I e-mailed him when I was looking into post-bacs and he was very helpful.
One thing to also consider is that post-bac curriculums are not very rigorous--usually it's just two science courses at a time. This is because many post-bac students were non-science majors or so far removed from school they have a lot of difficulty managing two basic science courses at a time. Generally you just take gen chem, o-chem, physics, bio, and maybe biochem and/or calculus. That's not many classes to boost a GPA with.
A post-bac is really for people that already have an overall GPA close to what they will need to be competitive for medical school. My GPA was a 3.3 or 3.4 in undergrad and went up to a whopping 3.48 after getting a 3.8 in my post bac. I did improve my science GPA what I considered a good amount-- I think I went from a 3.2 to a 3.5 or something like that. I had quite a few extra units over the minimum from undergrad (both total and in the science/engineering realm), so that made my GPA increase more sluggish than most others, but that's something for you to really keep in mind--it takes a long time to bring up your GPA, even with all A's/A-'s. I only got up to a 3.5 (actually probably 3.48--didn't break the psychological barrier), which is below-average for medical school matriculants. I was fortunate enough to do extremely well on the MCAT and that probably made the world of difference in my application. But you won't know how you'll do on the MCAT until you've pretty much finished all your coursework, and you can't assume you'll do well on it just because you want to (every pre-med says they plan to "kill" the MCAT).
Just to rehash--a post-bac is really for people that just need a dedicated and structured program to take the pre-reqs for medical school, and maybe a modest boost in their science GPA. (Obviously if you have no science courses and get a 4.0 in a post-bac, then the "boost," or rather establishment of an sGPA, is more than modest). It is not a reliable way to boost your overall GPA, because if you do the math it just won't get boosted very much--you're not taking enough units as a post-bac.
If you were to re-do an entire four-year degree and get a 4.0, averaged with your 2.5, you now have a overall GPA of 3.25. That's well below the national average for MD/DO matriculants, though maybe you'd get the benefit of the doubt in that case because someone who manages to get a 4.0/near-4.0 when doing a second bachelor's probably really has matured/grown up, and proven that they can handle the rigors of medical school.
I really don't think SFSU has anything to offer you. The community college route (if you have to choose one over the other) is probably the better option--it's cheaper, it'll let you use DrMidlife's advice and start slow and make sure you can handle things, and then allow you to transfer to a four-year. But you should really apply to a four-year college. Illinois has plenty of in-state schools--apply to all of them. Especially your alma mater--that's probably your best shot. You won't know if you can't get in unless you try. And you can always appeal. And you can certainly e-mail your alma mater and ask them for advice.
Unless you establish residency in CA (takes at least one year, maybe even two in CA), SFSU just doesn't make sense to me. And even then I'd still recommend you do a second bachelor's in place of a post-bac (though you could at least get federal loans for the post-bac, so you in theory could go into that program, complete it, and then enroll in the second bachelor's program to take more units/potentially get another degree. That's the only way I see SFSU making sense--but in that case it's still really no better than going to an in-state school now (since you already have residency in IL) and just doing the second bachelor's now. Plus, you'll want to keep your Illinois residency so you have a better chance to get into IL schools (even the private schools have a Midwest bias).
Ultimately it's totally up to you what to do. But I really think the advice DrMidlife gave you above is spot-on about what you should be doing. While I'd love to say if you work hard enough and want it bad enough that you'll get in, the honest truth is that the odds are very much against you getting into medical school with your 2.5 GPA. The vast majority of people in a similar situation would not be able to pull it off, and I really think your best shot of getting in is taking DrMidlife's advice. I think it's still sort of a hail Mary, but if you get a near 4.0 (easier said than done) and apply pretty much everywhere (MD and DO--I agree to pass on the Caribbean as it's even more risky these days) then hopefully at least a few schools will look past what would still be a mediocre overall GPA and look at a (potentially) very strong second bachelor's GPA and science GPA.
