nztim:

 

Now I have that out of the way the technical solution.

 

• I would not go down the mesh path all devices will be on the same Layer2 network as yourself, therefore occupants of the secondary dwelling can cast to your TV which is far from Ideal.

It could be done if using a router that allows multiple networks. For example the Synology routers can do multiple networks with different subnets and isolated. But I still wouldn't share my connection