[@1Charak2]
Oh, I'm sorry. This is something I've done a fair bit of consideration for but didn't throw much in other than

[quote]Accounts from the Revolution and the wars that followed it tell tales of 'a certain Marianne,'[/quote]
Because normally there's a Legend section to these Fate CS. I tried to keep it a little more minimalist for a Deviation section since the format seemed fun, focusing on the post-death strangeness and amalgamation of minor persons that makes the Spirit what it is. I was too vague or there has been some misunderstanding and I'll try to clear things up so please forgive another long winded post.

The least intrusive explanation for the Servant is that in the legend enabling alt-history of the Nasuverse, at least some of the folklore was telling the truth about a Marie-Anne that otherwise didn't make it into the books. Treating myth as misunderstood alt-history fact seems pretty par for Fate and doesn't intrude on any of the many people from the era that, like you say, have their own business with the Throne. You want them amalgamated with a living person, and I agree because that is already the truth. In this case the supplied person originates from apocrypha of the era. Someone who in reality certainly did not exist as depicted but in a world where every historical figure was actually a cute anime girl, may have.

If we have to cannibalize someone who was probably not just a legend in real life history, no biggie. There is a story that I'm pretty sure originated in tabloids which states that Paul Barras was who choose the name, and he did so because of the wife of Jean-François Reubell. Marie-Anne Mouhat. Not credible history, but she was in arm's reach of Reubell, at least. It leans harder into the identity being thrust upon someone after the fact, but her connections make her my gun-to-my-head choice for a Real Person™ that can hold the bag for us.

I'm happy to run with whichever floats better in your interpretation of the lore. I have a preference because for whatever reason I am happier to go crazy with explaining pure myth than trying to tie a legend to questionable history, but there isn't a whole lot of difference between the two in the grand scheme of things.