I will echo what the above posters said:
1. study and do well on your MCAT, I would not sacrifice a lower score in order to participate in summer research (if you can do both then great, but if not, then prioritize MCAT)
2. I would try to allocate a few hours per week on some volunteering or some other, non-intense, extracurricular -- something you can point to as evidence that your summer was productive (aside from just studying)
3. research can be an important part of your application, but it doesn't have to limited to a summer project. In fact, it may be more rewarding to engage in research long term (for a few semesters, for example) so you can see a project play out over time (plus get a solid letter from someone who you worked with for a while)
Good luck studying.