So the setting collection modification by allowed groups is meant for staff and users who are highly privileged, i.e. helpers maintaining wikis. Though it seems like I’ll need to fix the errors.
As long as collection modification by allowed groups is set properly high enough, collection and subcollections should only be created/modified by the topic owner (and staff). Topic owners should be able add ordinary users as maintainers to collections or subcollections, who can then add their own topics. I’m not quite sure of what your settings are to warrant separating them.
Two collections cannot link to the same topic. An error should be displayed when attempt to link a topic already in a collection. Similarly, only one subcollection is allowed per topic. This is because the collection are all public and should only be made by topic owners.
Hmmm… I get the feeling that it might be better to make a separate plugin that’s just folders of bookmarks that can be viewed publicly or privately… “Personal Collections?” “Bookmark Library?” “Topic Playlists?” Or maybe I just extend this one. But the underlying code and philosophy would be quite different between the two features… It’s a bit unfortunate the name “Collections” is quite broad and can be interpreted in quite a number of ways.