So posts which go to one community in the group can be seen by users who subscribe to any of the communities in that group in any instance… if that makes sense. But they are still looking at the post from the instance which it was posted to, not a synced copy

This is essentially what I was suggesting. But in order for users to see a post, their instance has to have a copy of that post, otherwise the instance would have to load it from the remote instance every time someone tries to view it.