I'm not sure I'm following. So nip29 allows to fork a community? Does that mean you can "copy" a group chat under a new owner? And now if I'm not sure which of those communities will be around later, I post to both communities but people might be subscribed to only either of them? So they might reply to only one community?

Other than that, I don't agree with your take to own the audience. I mean, nostr is an open protocol so if you want to build such a tool, by all means do but to decide on a fundamental level to have either the one or the other ... I don't agree with that. Let the owner pick and let the audience know how closed the community is. But in the end, each community member can already see and archive all community activity, so how to wall that garden I'm not sure.