I think there's one main theme of the review comments - function names and variable names got brutally long with the persistent_untitled_document_file prefix. I'd suggest the following - dropping untitled, file and shortening document to doc only so the above becomes persistent_doc - I think this is sufficient and actually improves readability of the code. It would be best if this naming could be used consistently everywhere.
Maybe to elaborate a little more: - `untitled_doc` could be used for things that are common both for instant save and persistent untitled docs like the various config file dialog elements - `persistent_doc` could be used for things related to persistent untitled docs - `instantsave` for instantsave related things (which I think is the case already)