If you want to seek status based upon your marriage to a permanent resident, yes, that entails marrying the permanent resident.
I'm not going to speculate on what will happen as a result of your overstay if you return home; even if that does not create ineligibility it could create problems upon reentry, and your nation of origin may be relevant to an analysis. If you want somebody to tell you what the consequences will be if you return home, you should consult an immigration lawyer. For that matter, should you attempt to remain in the U.S. based upon your marriage to a Cuban national under the Cuban Adjustment Act, it would be sensible to discuss your plan with an immigration lawyer to make sure that you meet all of the requirements of that Act.

