So is the implication that the red line from Yachiyo is from when she was Kaguya, specifically from the same time that the line for current Kaguya comes from?
^Yeah, the idea is that Kaguya only became jealous and had sex with Iroha because she saw that Iroha already had the line with Yachiyo. It fits in with the other bootstrap paradox elements of the movie, too.