Who said kidnapping? Don't know what happened to Sr Lucy after 1957. She may very well have died. Lying is ,well, lying. There are big ones and even bigger ones. The point is to hide the Truth, which has been done. Is there a limit to the Devil's lying?
Yes, you are right- Modernist Consiliar Rome benefits.

Suppose Sr. Lucy died in 1957.

In order for your theory to even be plausible, I must accept the following as fact:

1) Nobody at the convent was aware Sr. Lucy died (or, they were in collusion with the old “switcharoo,” and maintained their silence, or, they were threatened into silence (which none have ever maintained);

2) Nobody at the convent, or elsewhere, recognized the new Lucy was not Lucy;

3) The fake Lucy would have to not only be a 24 hour/day role, familiar with Lucy’s mannerisms, etc, but would have had to sacrifice her own life to play the role (for nothing in return except a personal interest in seeing Fatima subverted);

4) Lucy’s family was involved in the fraud by saying nothing, or simply did not notice their own kin was replaced by an imposter;

5) All the Masonic Vatican would have had to do would be to order Lucy into silence; it would not be necessary for them to replace her with s body double for public appearances;

6) 60 years passed before some obscure sede invented this hair-brained theory.  A normal person would/should contemplate why that is.  It gives every indication of someone inventing a solution in order to make Fatima fit into their own narrative.

If someone would have told you about this theory when you were 16, you would have said that such a person would have lost their marbles: The pre-conciliar Vatican inserting a fake Lucy to subvert Fatima, apparently totally unconcerned that anyone might notice.

Not buying it.  Not by a long shot.

1 and 2) Who said the convent sisters  didn't know sister Lucy died? Who said they weren't threatened to silence by the Bishop or higher? If a secret is to be kept, what better way to do it than to be behind a cloister, with  a proctor present for your bi-yearly behind the grate visits? Seems the family of Seth Rich is good at keeping secrets, too. Ever hear of the Manhattan project? Being threatened with death can be quite convincing.
3) The fake Sr Lucy is perported to be a real nun in the convent and possibly a relative of the Real Sr Lucy herself.
I think they all knew (in the cloister) How could they not? Do you think they would call a press conference?
4) The family was silenced- simple.
5) Yes, they DID need a new, smiling Sister Lucy to validate the "Sprngtime of Vii", not the dour Sr Lucy that has seen hell and Our Lady crying about the apostasy in the Church. They needed a Sister Lucy that was ok with John XXlll saying the third secret " was not for our time" and that JPll's consecration of the world was "accepted by Heaven". They needed a Sister Lucy to play down the upcoming horrors in the Church  (here we are) and dote over Vll Popes who disseminated lies about the Fatima message. In other words, they needed the real Fatima to go away to advance the diabolical.
Otherwise, I see your point as most people believe as you. I just have a problem with my lyin' eyes. As a nurse and a portrait sculptor , I am positive these Sr Lucy's are  two different people.

My main problem is that a WHOLE lot of unsubstantiated presumption is required to make the theory work, whereas this am habitually resolved not to let my conclusions jump ahead of the evidence.