On Screen 39.3, you can just put a "1" in "covered for all months."  That field doesn't care what type of coverage they had.

On Screens 39.2 and 39.4, use the coverage start/termination dates to match the 1095-A and 3895. i.e., if they forgot to cancel CoveredCA (like one of my clients did), it should not be an issue that they are double-covered with medicare.

If you have married couple where 1 became eligible for MC mid-year (again, like one of mine), you'll have two sets of each form.  Just "add" (middle window on the left) another form to each screen and input the matching data.

If you have MC premiums to report (and hopefully deduct), they'll go in one of the following: 14.1 (deducted from SS), 24 (if self-employed), or 25.