grammatical role of "using piano keys"

Question

You just heard "Ebony and Ivory" by Paul McCartney and Stevie Wonder. This is a song about racial harmony using piano keys. The black keys on the piano are ebony, and the white ones are ivory (from a textbook)

I have questions about the bolded part.

Q1.
I am curious about which grammatical role "using piano keys" takes.

1. an adjectival phrase modifying "a song"
2. an adverbial clause which can be changed into "as it uses piano keys"

Q2.
If the answer 2 is right in Q1, what is the antecedent of "it" in "as it uses piano keys"? "This" in "This is a song"? I assume this because if an adverbial finite clause is reduced into an adverbial nonfinite clause, the adverbial nonfinite clause shares the same subject as the subject in the main clause. Is this right?

Many thanks in advance:)

Answer 1

This is a song about racial harmony using piano keys.

There are different types of adverb / adverbial phrases. I would say that "about racial harmony" is an adverb of purpose, because the purpose of the song is to convey a message about racial harmony; "using piano keys" is an adverb of manner, because that is how it is done.

If you omitted the point about the meaning of the song, and simply said "this is a song using piano keys", that would just mean it is a song played on piano, and that really isn't so remarkable - lots of songs are played on piano. What is remarkable about this song is that it uses the black and white keys on a piano as a metaphor for racial harmony. So, this adverbial phrase is not really acting on "the song" - it is explaining how the purpose of the song is achieved. The order of these two phrases is essential to convey that.

Subject (the song) > purpose of subject (message about racial harmony) > how that purpose is achieved (via a metaphor)