Without a 1095-B, how is a preparer to know how many months the taxpayer was covered by Medicare? This discussion is overthinking and underthinking the question.
Being on Medicare does NOT automatically mean the taxpayer was covered all year. Form 1099-SSA does NOT indicate all the information needed to verify coverage for all 12 months.
Now my rant... As long as the IRS continues to grant reprives, move deadlines and ignore all the rules and penalties, there is no reason for anyone to do more than a cursory job of due diligence on this matter. The IRS does not know how and where things are at now, nor for the last 3 years, nor where they are going, how in the world are they going to police all these.