"How much does commuting cost into Paris as the job would be in central Paris?"
If you work in Paris the culture is to live in central Paris or as close as you can. So it would be a Metro or bus ticket at most (11.60euros for 10)
"What are the french academic standards like compared to ours?"
Children are expected to conform. There is much less tailoring than in the UK hense my choice to send my dd to a bilingual school.
"Would we be able to go for total immersion into french school for the DC or would billingual school be necessary?"
At least for your 9 year old and 6 year old I would recommend a bilingual school to embark on an immersion year because of the point explained above.
"Are all Parisians completely chic...?"
I think people in Paris tend to be more chic, simple and classic and in Blighty more cool, experimental and funky... but tbh there is a big ethnic mix in Paris so there are lots of styles going on.
"I will CAT you"
Good luck - If you do find yourself buying a one way ticket to Paris we really must meet for coffee and have a big big chat!