It is probably because of Rachel father, the western Duke probably saw use in Ariana like marrying off to fulfill his goal. I think in Ariana past life she got married to a merchant against her own wishes. If Ariana had stayed by her father side, the western Duke wouldn't be able to control Ariana to benefit him.
Exactly. They also were suddenly able to question the maids that worked there before. Even IF for some reason you're not able to plant spies, why not ask someone who used to work there like you did now? Or build a relationship to someone who already lives/works there? I'm glad he corrected her, when she was going on about how all of this is her fault I almost lost it...

I’m still at ch 28. If the mother hated Ariana so much to the point of killing her, why not just leave her with her father?? Like save yourself from the stress of raising a child you don’t love. And to The Duke of the East, for 18 years you were totally oblivious to what your daughter suffered through? And you claim you love her? You can’t see her because of politics?? You’re a freaking Duke and she’s your daughter?! And your only child. So technically, custody should lean more to you because she’s your only direct line heir. And you didn’t even think to i dunno plant a spy in there just to see how your daughter’s doing? All I see are lame excuses