Advice on WP site structure using multisite

1 – I cannot say if it’s the best way or not. I can say that I don’t have a better idea, so if it works, then I’d say it’s the best 🙂

2 – You found a plugin

3 – Not that am aware of, but you could possibly create a plugin that adds the menu to newly created sites via https://codex.wordpress.org/Plugin_API/Action_Reference/wpmu_new_blog