I'm not worried about such a thing, this is a common news cycle topic in Serbia whenever you need to draw attention away from something.
I'm afraid I don't really have any surviving documents as this was a few months almost a decade ago, something that ended up being more or less irrelevant in the general course of my life, so I haven't considered to keep any documents.
Upon further recollection I believe that the contract was in my actual name, the fake name stuff was overstated in my prior comment as it's been so long since I thought about this, it was used only for a few internal things and is to my knowledge legally irrelevant to the French government and to the actual Legion who if they have kept any record of me have certainly done it under my actual name, unfortunately there is a good chance that they just refuse to release any documentation in which case I think worrying about it might be a moot point and preparing my own proof is what I should focus on just in case.
What I do have is some souvenirs, like an official dated photo book of my regiment, that places me there at the specific time, additionally I am going to try to get passport information that corroborates the duration of my stay outside the country, affidavit of my time there, another signed one by my sister as she knew my intentions at the time and can confirm my timeline, along with all of the unanswered emails and short refusals from the official Legion communication channels seems like the best way to go about this. I am also thinking about looking into whether I can get a 'in good legal standing' confirmation from the French government as a foreigner, I have to present one from my local police as well anyway.