So, yes, the father/duke is the one that asked the wizard to turn back time. Why? Because Cosette is his daughter, but the soul inside her is a demon's. Cosette was born stillborn (so never actually lived) and her mother apparently made a deal with a demon to get revenge on the duke and all that happened to her. The first wife still died, but a demon took over Cosette's body. This means that after Keira died in the first timeline, the actual saintess that the oracle prophesized was dead. No other spirit users would be around. The demon used this opportunity and raised hell with a demon army and the world basically went up in flames. So the father/duke went to the magician to have time-reversed and leave Keira with her memories so hopefully, she'd stop this from happening. Keira is the duke's daughter, for sure. And from the sound of the spoilers I read, she is the real saintess spirit user. The only reason the demon!Cosette can put on that spirit display is because of some shenanigans with demons using a blood relative of the spirit users (Cosette's body) to manipulate the spirits. Though I will say this spoiler was less clear, so take it as speculation as even I'm not 100% on how a demon in a spirit user's body could control/show spirits to "prove" they are related to the duke.