I should also say that it works as expected and resizes the sheet on Catalina. It does not work as expected and has this error on Big Sur.

Same here on macOS Big Sur beta 4 with Xcode 12 beta 4.

I am working on a cross platform app and using the app lifecycle across all platforms would be great. Since I can’t do that, my current plan is to keep using the new lifecycle on macOS, but fall back to iOS 13 way on iOS until there is a way to accept CloudKit shares in the new lifecycle.

There is only one CloudKit container per app. (Well, I guess there can be several, but mostly you work with one.) That container, though, contains both private, public, and shared databases. It is possible to mix and match the different database types. I am currently working on an app that uses all three database types. So yes, it’s definitely possible.