If I kill the MCAT, do I even stand a chance?
Not with those GPAs.
Should I take science courses and nail them first?
You need a post-bacc. It can either be DIY or in a formal program through a med school, but it's necessary to boost your GPA.
What could I do to improve chances?
The traditional ECs are clinical and nonclinical volunteering (or employment), shadowing, and research. I don't know much about the ECs for chiropractic school, but I imagine you're already fine on clinical work given your training and employment. And I would forget about research if I were you: it's only a major determinant at a few research-heavy schools. Shadowing will be very important and no, your clinical experience as a DC won't satisfy that. The idea behind shadowing is that you get to see multiple clinical and nonclinical aspects of a physician's day to determine whether it's really for you, and med schools generally don't consider that goal accomplished by shadowing or work in other healthcare roles.
What you really need to do is prove to adcoms that you've thought this through, you understand what you're getting yourself into, and you're really committed to it. Specifically, adcoms will be concerned that you put in all the work for a doctorate, worked for just two years, and then decided to jump ship. How do they know you won't do the same thing again? You need to prove to them that this switch is legit and doesn't mean you're just flighty and restless.
I suggest you do some shadowing first to figure out how much you're willing to commit to this career switch. If shadowing reinforces your desire, then get busy with a post-bacc. Good luck!