From the experience of someone who has built a fair few websites with Grav, and maintains several plugins and themes for it, I'll try to answer points 1-7:
Whilst the Admin-plugin contains just about every setting, you can change what is available for what user by creating groups with specific permissions. Editing a Page in Grav is a very similar experience ...
OctoberCMS for the win, if you think it's too much developper oriented, take a look at static page plugin, or content editor plugin which allow easy edit for clients.
Every people that take time to understand it's concept never go back
I think just yesterday I have recommended this opensource webapp for another question but it fits the bill perfectly for your usecase (that is if I have understood it correctly):
This is an authoring tool and not only does it allow all the usual animation and stuff it has this very cool feature for polls which might be useful in a classroom ...