I believe he got this detail right in both ways; in that firearms were the most important weapons, and also the main characters would have done their very best to ignore that fact.
Was France using other people's currrncy back then ?
It was very common before nationalism and the standardization of currencies. I read primary sources about conquistadors and the contracts financing and supplying the expedition might involve a dozen currencies because each trader supplying the wood, food, animals, etc would work in their own preferred/local currency.
Not sure about the Occitan; IIRC Eleanor was considered a queen in her own right as rule of Aquitaine, not a duchess.
Are you sure you know what the coin paid you is made of? A merchant of the time wasn’t. Those who care not to be scammed have never found it simple.
If you want to buy something worth 6% of a gold coin, whacking off an edge of one is a weird way to do it. You'd need a scale handy.
https://www.goodreads.com/book/show/13330922-the-black-count
which arguably served as a template for a famous novel which also features a count in the title.