So, I don't believe she was still alive. Why? Because Our Lady would not have subjected her to the Novus Ordo Mass, etc. ... a consideration lost on them at Fatima Center.
His objection that it would be hard to get an imposter to become a cloistered nun and live that life 24/7 is weak. They could have just enlisted someone who already was a nun, or else not had her actually living in the convent except for those occasions when she needed to appear.