I have decided that for maximum parallelism and impact actually it should be replacement-gege who is getting into Jiang Cheng's head (by Jiang Cheng choosing him over Wei Wuxian to experience the inside of him) and seeing how he lost his core and then telling Wei Wuxian about it ONLY on being thoroughly pestered because he couldn't really betray sect leader just like that! And then he should follow that up by being extremely apologetic and bursting into tears when Jiang Cheng confronts him about it. (<- he is immediately forgiven) (<- "look what a manipulative traitor he is lan zhan!" says Wei Wuxian while replacement-gege almost throws up from misery)









