It has been a while since I read the novel and I didn't read the whole novel so I only remember a few stuff
Ishakan is going to kidnap Leah when she was in a carriage on the way to marry the margrave, he brought her to his kingdom and married (or hasn't yet) Leah. One day she had to go back to her kingdom, if I wasn't wrong she thought that Ishakan was dead.
The brother lust after her, even though he was already married to a lady, he still wanted Leah. He expressed the feeling to his mother and his mother, who's some sort of a witch gave in and let him 'have' her. The king, Leah's father was under the queen's spell this whole time.
That's how Ariadne was fooled. If this is the intention of the artist (to show how someone could be fooled by a manipulative abusive cheating bastard) then they did well.
Like I'm currently rereading and in Chapter 36 there's a letter from a young lady saying 'your passion of waiting outside of my window frame all night until the morning dew falls has mysteriously vanished' really tells that Cesare only behaves like that when he hasn't gotten the lady's heart yet. If he had gotten what he wanted, he left them to find another 'quest' to be conquered. Remember that Cesare wanted to have Isabella's heart too until he found out about her flaws and Ariadne entered the picture.
Zeno sealed Jae-ha, Kija, and Shin-ah not because it's the end of their lifespan. That chalice is really important to Zeno not just because it's what gave them the powers, but it can actually extend their lifespan. There are a few theory I consider about this,
1. Zeno sealed the dragons so they can recover, both their souls and their bodies.
2. Three of the four dragons aren't going to die young this time around because the reincarnation of King Hiryuu has descended. The first generation of the dragons all lived until old age, the descendants died because King Hiryuu wasn't in their generation(s).
3. Zeno would release them back when they're ready or when the time has come for their release.
4. As Kija felt, Jae-ha and Shin-ah are still alive, it made no sense that the three dragons are dead or waiting to be reincarnated again because Yona would sense it. Zeno looked relieved because he sealed them before they died or lost themselves.
Again, this is just my theory. I hope it's not too far off.
From the look on his face on the last panel-- they really put the obsessive in obsessed