The problem with average per song is that you "use up" words in every new song, so all things being equal each marginal song has progressively fewer new words.
http://www.mdaniels.com/vocab/scatter.png
love your other ideas – hopefully can do them later.
In fact, if their first song used n distinct words and their second used a completely distinct set of words, but the second song was shorter than the first, their score would drop.
That would be unusual behaviour for a measure of vocabulary.