1. Not really sure, what is meant by Office 365 tenant
2. For now, some schools have their own domains and there is no structured organization.
But, yes all subdomains are going to be in the same forest.
3. Yes, probably that's what I want to implement.
4. All cities will be linked by VPN
5. I want primary DC at Headquarters and make secondary DC at schools.
I want Office365 usernames match future AD users.
Is it info detailed enough?