I don't think you'd consider the workaround you provide for reverting back to an external reader acceptable for anything else in Zotero.
We offer "Convert Linked Files to Stored Files" — to migrate to the seamless, recommended workflow — but leave the inverse to manual migration or plugins. That seems fairly equivalent.
If you could annotate on iOS, and then go back to the Desktop version and store the annotations back to the original PDFs for those of us that use an external reader, it would be fantastic!
Upload image file with the file dialog or by dragging and dropping images onto the dashed region