But the plugin doesn't need to know anything, all it needs to do is disable the video playing capabilities, so become a pure "audio player" (just like an audio player can't play video tracks).
Don't think lag is avoidable for very big video files, though the plugin could have a file size threshold, above which it wouldn't try to parse anything
Do you mean this FAQ? HOW TO: Enable/fix playback of various media formats in Opus?
It doesn't list the two codecs I see: MediaEngine and MediaSession. I guess the first one is #1 on that list?