The story opens with the MC's husband telling her that he wants a divorce, no real justification offered. She just decides to obey instead of demanding more explanations and confides in a few people to no real effect. (I might have missed it, but I didn't see any mention of where she's go after the divorce, probably because the author already knew that it wasn't happening.)

That's right, the divorce doesn't happen, but only because her husband supposedly elopes with the maid that she's close with. From that point on, the author starts political conspiracy tidbits, but the whole thing comes across as flat instead of thrilling information.

(They eloped to a foreign country. The maid was actually a spy. Maybe the husband is selling state secrets. The husband hated his father because of something relating to his deceased foreigner mother. [insert flashback/"when I was a kid..." narative dump here to explain preexisting relationships between all of the characters in the story except the MC]. Maybe the husband wasn't sleeping with the maid, but he's still ran off with her for some other reason.)

As for the romance bit, the MC's childhood friend Fee conveniently shows up just when she needs a shoulder to cry on. He's related to her husband through implied family relations that boils down to "MC's stepson's uncle." And he tries to woo her while she's probably (or ought to be) emotionally vulnerable at the occasions when she takes the kid to visit him.

I sure still don't know why Fionne is suddenly the prince.

her husband basically live his life with unsettled vengeance in his heart. It's true that their marriage was arranged, but he loved the warmth and serenity everytime he go home (albeit RARELY) that comes from his second marriage with FL. (he is described as workaholic to the core). However, his vengeance is more deeply rooted, so when Anne comes, he finally see the path to settle his vengeance. So, he abandoned his wife and used Anne as the stepping stone for his vengeance.

for me, someone with deep rooted vengeance won't easily change their goal. BUT, when FL was kidnapped by one of his men accidentally, he suddenly decided to treat her nicely and wanted to reconcile. He even do drastic measure of abandoning Anne (his ticket to his revenge) to prove his words, WHEN HE STILL HAVEN'T GET HIS REVENGE YET [For me, it's like author just wanted to add 'rival' for the actual ML.]

And of course, in the end... It was all a bagsh*t MISUNDERSTANDING (Japanese seems to really love this. A prolong misunderstanding that goes on for decades)