I think it would be nice if we could manually associate 'failed'/'errored'/'incomplete' downloads with the real corresponding video.
I think you mean: even though they aren't. Also, that used to happen to me until I updated PlayLater and now it seems to work fine for the most part.
I do agree, though, that being able to manually associate the video with failed/error/or incomplete sounds like a good option to have.
This has actually been possible since PlayLater was released. You can right-click any recording and edit the status.