It all seems pretty much open and shut.
Best case scenario, the real Sister Lucia died naturally, and for whatever reason, they enlisted another woman to play the part of Lucia, either (a) to have a convenient shill for Newchurch, (b) as good public relations, to be able to trot out a living Lucia on demand, or (c) both. Worst case scenario, they either imprisoned or, God forbid, killed the real Lucia, to be able to have the kind of shill I described above, to keep the "real Lucia" from denouncing Newchurch, and/or to keep wraps on what the Third Secret really was.
Being able to have "Lucia" say that the consecration of Russia had taken place, and had been accepted by Our Lord and Our Lady would have just been icing on the cake.