Hi! Writing here because @AntiGravity kindly informed us of this thread as we’re the authors of the MultiWorld plugin, that does exactly what you’re discussing here: manage multiple UWorld instances in-game.
The plugin supports a number of listed nice-to-have features, like Level Streaming and World Composition. At the moment, for technical reasons, the most requested missing feature is network replication of Secondary Worlds (they’ve access to networking features only through objects in the Main World).
The upcoming new version of the MultiWorld plugin adds support for capturing scenes from Secondary Worlds, this will make possible to create Portals between distinct UWorld instances (to be completely transparent: we’re not providing a Portal system with the plugin 
).