Happy to help where I can!
The payments already made won't change retroactively as far as I know, no, but will affect any future payments. I'm not well versed on french taxes admittedly, so you'd need to see if they have a tax treaty with the US or not. Did you have a default holding rate of 30% beforehand? As that can certainly take a large chunk of the transactions.
Viewing post in Itchio taking more than 50% of the payout?
I believe so--but one of the staff can clarify if I'm wrong on that front, as I wouldn't want to accidentally spread misinformation.
You'd get a bigger and more precise breakdown of the calculations when you initiate the payout I believe, since there's all sorts of external factors like payment processing fees and such. It's all quite transparent about where the money is going, I think!