If you got the interview, that means your GPA isn't, or shouldn't, be the limiting factor here out of your entire application! For the MMI, I used Rollin's book as well as practiced MMI questions every day. It helped that a lot of my friends were/are going for med school so I practiced with them as well. For the essay, I reckon they mark more on how eloquently you express yourself rather than the content itself.
Out of the 10 pre-req courses, how many do you think were "below" average? If you think they affected your application significantly, it might not be a bad idea to retake them, but also be a full time student this year because they look at the last 2 years, and if you take your pre-req courses again and add a bunch of fluff courses to boost up your GPA, it might be worthwhile. However, I do agree that the MMI is the largest part here. I suggest that you come up with some sort of structure to help yourself through each question in the MMI, such as a intro-present ideas from both sides of the argument-choose your stance-explain why-conclude, something like that. Of course, not always will you have a "Choose A or B" question, you might be asked what you would do in a situation and such.