If im not mistaken, Luceris and Lancelot will get into a relationship if the MC doesn't burn Farah room. If you chose that, then they wont.
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Thats the only way to prevent them to already be in a relationship, but from my understanding the romantic tension will still be there howewer since Lancelot cares about MC opinion, if you openly show your disapproval then he wont pursue a relationship.
And even if they do get together if MC didn't burn Farah room, you can break them up.