I think I found the C
C = Choiseul
https://en.m.wikipedia.org/wiki/%C3%89tienne_Fran%C3%A7ois,_duc_de_Choiseul
this is the guy who commissioned the painting that is the puzzle that Ellen is putting together in her video. He is one of the people in the painting.
https://en.m.wikipedia.org/wiki/Modern_Rome