On the collaborative use-cases, I wonder if there’s any merit to the idea of declaring API servers for editing records.
Let’s assume you’ve successfully shared the private collection with the intended recipients only. To support mutations, you could just declare in the record that foo.com controls the record. Then collaborators just submit edits via API requests to foo.com, which has been granted prior access to mutate the record.
I suppose foo.com could actually be the collection owner’s PDS, if the needed APIs are generic enough.