Check the following documentation:
https://documentation.meraki.com/MR/WiFi_Basics_and_Best_Practices/Extending_the_LAN_with_a_Wireless...
https://documentation.meraki.com/MR/Deployment_Guides/Mesh_Deployment_Guide
You'll see that it is indeed possible to have such a setup. The switch can help in the sense that the two central APs don't need to use airtime for their mesh communication. Note that this switch cannot be used for any client devices without introducing a router.
In your setup a client will be two mesh hops away from the gateway access point. This will reduce the clients' speeds to 25%, which is not ideal. There's also only a single mesh path.
In the buildings themselves, are you just going to use wireless clients? Or were you planning to put a switch? If you wanted to put a switch, you need to introduce a new router in each building. In that case, you may be better off looking at a purpose designed point-to-point or point-to-multipoint solution that behaves more like a black box link between the buildings. They typically use the 60GHz band for the backhaul network. In the buildings themselves you then have your switches and access points as you normally would.
