None of these will work properly. If you do copy-on-write where both the upstream server and local client make changes then the chunks you (or your local client) made changes to cannot be updated from the upstream server. However, neighboring chunks can still be updated from the upstream, which means you get holes in buildings pulled in from the upstream world.
> Right now, you can visit the Marketplace to pay Minecoins to download maps others have made.
Cool, that's the way it should work. A snapshot stored in the cloud that you can run in a server or locally. Before this existed people posted saves online that you could download and play.
> Add ways in-game to make a portal to someone else's server... and you're making something really neat.
Pretty sure this has been a thing on modded (Java edition) servers for a long time now!