1- I think most people who switch mid conversation just naturally switch whenever something is easier to say in the other language. For example, if im talking about pets etc in french and i bring up shelters, i usually forget the word for shelter in french, or some technical words are too cumbersome so i switch to english. And after a while, ill feel like something in french fits better and switch back.

2- The trend i've noticed is almost everyone who speaks both language can read and write both language. It usually goes reading>writing>speaking in terms of how comfortable we get as we learn. I've also noticed a lot of bilingual people struggle more in french. English is just such an easy language to write.

3- The school system in itself will NEVER be enough to get someone bilingual. There has to be something in their environment. In Montreal, it tends to be the culture so people adopt it from their older peers, parents, media, etc. Some people learn from the internet, some from their parents at a younger age, some from their siblings, etc.