Yuki Kajiura Live vol.#20 (June 2 - Aug 3, 2024)

And they released the live where it was performed for just one time. We won! 🥲

I didn't realize that. I regularly listen to that performance.
You should keep on mind that the main theme of PC has some layers, Eri was alone on that live so moonflower is easier to perform.
It's true that the main theme of petit cossette has some layers, but so does credens justitiam, which she still sings this song.
Credens justitiam only has a backing vocal which can be omitted perfectly. The main theme of PC has, at least, two tones and it has two layers of voices interpolating themselves (btw, Keiko is perfect to get the effect for Yuriko). Could perform the song just one person? For sure, but the result will be a bit... "thin".