I recently read the New Yorker article on this: https://www.newyorker.com/magazine/2020/02/17/was-jeanne-cal... which is very long and very worth reading and which I think gives both the "yes" and "no" sides a fair shake. Some key takeaways (my interpretation anyway):

-Yes, she looked startlingly young for someone her age in many pictures

-The extremity of how old she got is really surprising, considering that we can make distributional assumptions about age and that the existence of Jeanne Calment would imply that we should see quite a few other people at ages slightly less than hers and the next oldest is three years younger.

These are both surprising but not really independently so - it can hardly be surprising that someone who lives to an astounding age might look younger than they would be expected to.

Most of the other evidence, in particular her occasional slips mixing up father and husband have no real evidentiary value. Confusing relatives with each other is something that people with incipient memory failure do literally all the time.

However, I also think that the "two people" / "daughter swap" hypothesis, doesn't stand up to inspection very well. Specifically, the problem with it is that there is quite a lot of recorded testimony and recollections from people who absolutely would have known both of them, had no reason to lie, and were quite clear that they were different people.

Specifically in that article:-

Claudine Serena (or more particularly her mother and grandfather)

Freddy who was seven when his mother Yvonne died - can a seven year old keep up the pretence that his mother is his grandmother successfully?

Gilberte Mery who recalls the tradition of the promenades and the degree to which Arlesian bourgeois were constantly watching each other

The notary who administered Jeanne's and Yvonne's marriage contracts and then later carried out real estate transactions for Jeanne

Yvonne's husband requested leave from the army on the grounds of his wife's serious illness (and we have records of that)

Yvonne had a substantial public funeral, which would have involved viewing of the corpse by the very large number of prominent local people who knew her

I went into that article thinking that this would certainly be fake but came out thinking that faking this would have been completely impossible. There were just way too many people, many with no interest in perpetuating a lie, who would have had to go along with this - long before any possible motive about the world's oldest woman.

As to motive, apparently actual inheritance taxes would have been not the 34% in the OP but a mere 6% - an easy mistake to make if you don't have access to century old French inheritance tax law but a pretty important difference.

Incidentally, the role of Aubrey de Grey is very amusing in that article - it is pretty clear that he doesn't actually take the impostor theory too seriously but is using it as a tool in order to get his hands on the DNA sample which obviously makes sense given his interests.

To quote from the article: Either she had lived longer than any human being ever or she had executed an audacious fraud. As one observer wrote, “Both are highly unlikely life stories but one is true.”