Does it in fact have rhythm, melody, harmony, unity, or continuity? If it's too slow for any human to actually observe those qualities in it then I'd argue that it does not.

Are you talking about the composition or this particular performance of that composition. This performance may be too slow for any human to actually observe, but the composition is just a piece of sheet music you can read and judge like any other composition. This is far from the first performance of this piece, it's just that most other performances have been in the 20 minute to 20 hour range.

Do photons and electrons not exist, because we cannot observe them directly?

You can certainly play a recording of the piece at whatever speed you desire, and decide if it has rhythm, melody, harmony, unity, and continuity. Extremely slowing the piece down does not remove those things.

It’s playing a chord, so harmony exists by definition. It also has 600 years of continuity planned in.