Well well well

i feel like it has something to do with the plot later on

Doesn’t the guy know though? I thought it was clearly shown that he knew and that’s why he helped fake her death so that no one would question it. They can’t use another scenario because they didn’t divorce ig and his wife ran away with his brother. Those two would’ve gotten in trouble. He knew his wife and brother liked each other and didn’t mind it. The fake death also helps so that the son doesn’t get curious and search for her had they told him she were alive

No I don't think he knows. He's mentioned her death before also noting smth like "if she's watching over, she'd be disappointed that he hasn't been a very good father." She's the one that helped her fake the death and escape

If you check the comments from the earliest dates, you’ll see people confirming that the duke does know from what they read as well. To show details if you forgot:

Chapter 2 quotes Dieline: “I have kept my promise to the duke, now I’ll go find hygel. I have no choice but this. Ambrosia needs a successor.”

Chapter 10 quotes Ethan: “Even if Dieline comes back, and slaps me for neglecting my child, I have nothing to say.”

Chapter 15 quotes Ethan thinking of Claude: “The only blood relative left by my younger sibling (Hygel). Who left for the well-being of the family.”

Chapter 16 quotes Ethan talking to the FL: “To the extent that I thought it was a gift that Hygel left me, Claude is a precious child.”
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All the hints show that he does know about how his wife and brother loved each other and had Claude. He never slept/touched his wife, so he knows that Claude isn’t biologically his. He knows that hygel and the wife left and likely helped cover for them with the story that they died. Realistically it wouldn’t be possible for him to think they’re dead when he likely could just investigate and find out they aren’t with his power and influence. He’s the one that’s able to put a stop to rumors or people wondering what happened.

That's really speculation imo, he knows Claude isn't his child but it's unclear if he really knows she's alive or just turned a blind eye to her "death".

It's also hard to tell with the unclear translation but in a lot of east asian cultures, it's pretty common to say so-and-so will come back [from the dead] just to lecture you. I heard it all the time and that's how I took it but you're right, it could be alluding to him knowing.

Still, it's sour to know he's endgame when her supposed best friend, whom she doesn't even contact or check up on anymore??, was once married to him!!! Just bc there was no "love" btw them and they allegedly never slept together (bc we don't know for sure at this time) doesn't mean it's not scandalous. He's fine but it would realistically ruin her reputation!! Summarize this plot and who in their right mind would say it's an ideal romance? Honestly?

But what about Dieline mentioning her promise to the duke is basically giving/leaving her kid as the successor? It’s a pretty good guess that he knows and let her go with his brother. It just would not make any sense whatsoever for the duke to not have the capability to find out about her “death.” He would have easily known she isn’t dead, he’s got spies and everything. It makes way too much sense that he would go along with the fake death so he can fool every other servant and Claude himself. To prevent Claude from asking or searching for her.
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Anyways that last part is a personal preference. I don’t mind at all since it’s clear the best friend doesn’t give af about the duke. FL knows that her best friend is living happily with her new family so there’s little to no guilt about getting with her friends ex husband. You forget that people in those times are very used to arranged marriages and knowing about having no love involved.

Also I would say it’s likely he didn’t touch his wife from the fact that his wife really seemed to dislike him, it’s been mentioned before that he’s incapable of giving his heart to anyone and with his way of thinking it’s likely he was not interested in anything romantic. He also knew about his brother and his wife’s love for each other and he cared about his brother a lot, so it’s likely he wouldn’t want to do anything to his brother’s lover.

I think you also forget that not many people actually know about the mc. At most, the higher figures know about the fact that she’s powerful, but they likely don’t know every little thing about her life like who she’s friends with. People in these historical stories often remarry to continue having an heir or other reasons so it’s not unheard of. I also highly doubt the FL cares as she has so many other important stuff to think about and lived multiple lives. The only thing that matters is that the duke and FL are mutually gaining feelings for each other. I’ve read tons of these historical stories where the circumstances are worse, but they always find a way to be together, reputation is the last thing they worry about lmao. Other plots have had the mc marry a ML who’s previous wives all died, arranged for money, marry to a man that was previously married to a princess, etc. In your eyes it isn’t an ideal romance, but an ideal romance is boring if you can easily tell what you want to happen.

I'm not following why you think her promise to give an heir is admission that she told him she was faking her death. It's a duty for them to provide an heir whether they like it or not. And why would she go crying to MC for help if he already knew and would help cover her tracks? MC's also a powerful magician. I think it's safe to say she can avoid a couple of informants when she's even ghosted her disciples.

And how exactly is her best friend "living happily with her new family"? Remember the brother ran away first out of guilt from the affair and she went to look from him. Presumably she found him but we don't know that for sure. They've left all their friends, family and CHILD behind. Happy is the last word I'd use yet MC isn't even shown checking up on them?

Lastly, I said "realistically" as in historically speaking. In doesn't matter if it was an arranged marriage or not, the smallest alleged scandal would ruin a noblewoman's reputation and have them shunned from high society. The men ofc would get off free but that's beside the point.
MC mentioned the crown prince once fancied her so the older nobles definitely know about her and she's never hidden her friendship. It could be easily dug up. Heck even today, tell anyone that you've fallen for your best friend's ex husband but it's "okay" bc they didn't love each other and watch all the raised eyebrows judging you. Just bc you've read other stories with worse premise doesn't mean this one's okay. That's like wondering if red cherries are more red than red apples. They're both still red.

You must have forgot why the friend was crying and asked help from mc. She didn’t ask for her help to help cover her death. She asked for her help to watch over her son when she leaves because she knows her friend has the magic to be able to contain the curse. That’s the whole point of mc being the nanny in the first place because she’s the only one who can help with the curse.

If you read the latest chapters you would know mc DID check up on her by reading the letter that says her best friend literally had a whole new kid with the brother, a daughter this time. Hence why I said happily because the mc thought so. Did you miss that part?

Regardless, even if you don’t like it and don’t think it’s an ideal romance, it will work out whether you think it’s possible to or not lmao. I gave you examples of other stories where situations were similar or worse but they all turned out to be perfectly fine with happy endings and the two main characters ending up together regardless of who judges. I never said anything was okay?? I said that it will likely turn out okay, even if you think it’s insane. Or are one of those people who judge.

Also using my critical thinking skills I believe her saying “keeping my promise” to give the heir to him means it was an AGREEMENT of both parties. Aka there’s hints something he gives her in RETURN as well. Aka freedom to leave. I just don’t understand why you’re so adamant about the point of him not knowing despite the many hints and literally everyone who reads this agrees as well that he knows except you lol.