Medicare is "government-subsidized", not private insurance. MA does not require you to enter any ID numbers.
Yes. Per the State of Massachusetts, if you did not receive Form(s) MA 1099-HC, enter the name of your insurance carrier or administrator and your subscriber number from your insurance card.
Since you were enrolled in Medicare, you automatically meet the requirements for Minimum Creditable Coverage (MCC).
Medicare is "government-subsidized", not private insurance. MA does not require you to enter any ID numbers.