To be honest I think that Issue28 is likely right how this would end in real life with possibly both Jamie and Liz getting seriously emotionally hurt.
This is a game though and mostly an overall feel good game. So I suspect after several Millie tantrums, possibly her even working for a bit with enemies of MC she will somehow see the light, break through her false illusion with some help from possibly one of the older females (Not sure Wendy with her abuse background would be the one that could relate to her and by telling her of her own past open Millie to her own situation or someone else) and Millie will slowly heal and get in a more healthy relation with Jamie and Liz and possibly MC. This game is partly about that healing in the end and that is also what the Hearts Cult is about.