Should I get post-bacc in Mathematics or go straight to Computer Science Research?
Tough question, mostly because it seems it would have a lot to do with you and your overarching goals.  Is the goal to advance your career or is your goal to get the Master's degree and write that paper and publish?

It's tempting because you have the free course option right in front of you but you can take classes in machine learning from Coursera (for instance), for not much more. So if your goal is simply to move your career ahead school is likely not required at all.

But if you want to teach and publish and get that piece of paper then absolutely you'll need more school. Take undergraduate courses and get that GPA up and then apply to grad school.