Hi, I have a French parent, but was born and raised here and my other half is French. I can give a view on some of your questions.
We spoke French all the time to our eldest, except when we were with English family and other English speakers. She took to bilingualism very well so we thought, this is easy! We didn't push reading or writing however, but her spoken French (and English) was excellent. It was much harder once she was at school as she obviously struggled to explain her day in French - I remember lots of "il était naughty" and "on avait carpet-time"! I decided to push for her to do Spanish rather than French at High School as she was a good linguist and blithely said I would do GCSE French with her. School was extremely supportive and helpful, but it proved impossible time-wise and trying to compensate for the fact that we hadn't kept up her reading and writing. She was also then not very interested and has gone to full on 'answer in English' mode. I have sympathy with this as I was exactly the same!
We began well in the same vein with our youngest, but she wasn't at all as good and really struggled with English, resulting in me being called in to nursery and having her hearing checked. She was fine, just not as good as the older one and in hindsight, we should have kept going with French at home. The kids would also not speak to each other in French, so the younger one is also now in 'I understand everything you say, but will only respond in English' mode. I am therefore requesting she does French at High School, and I'll help her with Spanish.
According to their dad, their French is native sounding, but then his family say he speaks with an English accent after 20 plus years here so who knows! I've always had an English accent on the other hand.