@Mimi I'm suspicious that this might be a ploy rather than negligence. That he's doing
exactly what Chester is. Letting the walls close in around her so she has nowhere le he better have a plan in theft to turn, then extending a 'compassionate' hand for her to grasp onto.
He just wasn't expecting her to break in the way she has. The last time he spoke to her she looked like someone who had given up or at least was in a very fragile mental state. Despite being in poor health she's stopped eating, stopped putting the heater on despite how the place is too cold for her,
she's stopped looking after herself. That should have sent alarm bells ringing.
He might be preoccupied with Midnight's betrayal but even after he's said he trusted her
he still didn't lift the house arrest. Confining her to the annexe might keep hurtful gossip out of earshot, still he could have said you're not under house arrest but I want you to stay her and recuperate.
Could have summoned a doctor immediately for the woman who has a serious illness. He spoke like he was expecting Marine to report to him later earlier. Maybe Marine was already reporting her health regularly to the Duke because he was concerned.

Sod the apology that doesn't fix what's been done, or her now destroyed reputation. He better have a plan in the works to absolutely
destroy the instigators of this.