The first dialog tells you that your file is not seekable. It does not matter that you created it with VLC and that it plays in other players. Shotcut can play it too, and players are not editors that require precise seeking. If you choose not to convert or do not wait for conversion to replace your file, then Shotcut treats a non-seekable file as a “live” source. The Source player actually says “live” where it usually shows the duration. A live source is like capture from a device like microphone or camera and has no inherent duration. You can still add a live source to a playlist, but you must supply a duration.