I can see the motive of the Vatican in bumping Lucia off. I don't see what they gained by installing the imposter.
Why not just take Lucia's writings, burn what was inconvenient and forge writings that backed Vatican II.
The imposter Sister Lucia did very little. Even the confirmation that Heaven had accepted the consecration was not made on video. If you have an imposter why not have the words come directly out of her mouth.
If the imposter does nothing for the modernists then it is purely a downside risk installing her. She could be exposed or expose herself fearing hell for her deception.
In 1959 Sister Lucia was quoted in Catholic newspapers calling Father Fuentes a liar. The whole newspaper can be found online and the article in question was the headline one.
Now, sure that could be the fake Sister Lucia, replaced between 1957 and 1959 but I would like some proof of that.
Looks to me like the same person after dental work. I fail to see how the Church finds a close match to her looks and convinces her to take on a false ID and live in a cloister for the rest of her life?