Only remaining question is how to formally record when we leave France as there's no border post to get a stamp into Italy. I can find no information on how to do this; any advice welcome otherwise we'll just wing it and hope photos with time stamps will persuade Dutch officials on exit that we haven't overstayed.
I would imagine that there will be little problem given your (presumed) situation.
To improve the veracity of timestamped photos, I'd suggest emailing the relevant photos to yourself. Perhaps with an accompanying position on Google Maps.
Perhaps also have the French visa documentation (in electronic form) emailed to yourself and on the phone. Sometimes being heavily prepared will discourage too much interrogation. A decent map of your route is bound to be distracting too!
Allow plenty of time for the exit process in NL, be patient and smile. A lot. If things aren't going your way rather than arguing directly, try to ask questions such as "are you sure?", "can you check that, please?" etc.
Your situation will not be "normal" and may invite closer scrutiny. Being calm and pleasant can help enormously. Being prepared even moreso. It took me a while to learn that lesson in NL!
Edited to add: I'm just a randomer on the internet.