i don't think that lady ga-on was the one who planned against her. She might've been a pawn of either one of those two concubines or the both of them who are planning on slowly plucking out the flowers in the Emperor's Garden (figuratively, lol I'm getting emerged with these kinds of words so pardon me) and leaving the Empress and Lady Lyn for the last.
It really feels as if they tested the waters because of the rumor that Lyn and the Empress are getting along. So they wanted to frame the Empress but it backfired so they probably blackmailed that concubine to be a scapegoat. And I'm also thinking that they might be colluding with that guy that's said to be backing up Lady Lyn. ( ̄へ ̄)