Actually, it doesn't work that way, and it's kind of insulting for it to be described that way too, especially after he showed that "Why yes I do have a grasp of politics".
He wouldn't be King, he's be Prince-Consort. A title that has no actual power in itself except in so far as sharing the bedroom with royalty. More importantly, this only happens if Isabella inherits the throne, which is pretty unlikely as she's rather far down on the inheritance list. If this doesn't happen? He simply retains whatever titles he already holds.
I get the feeling that Karin kind of got poleaxed by the fact that he actually knew what was going on, and tried getting back on script, and when he deviated again, she just snapped and ended the discussion with what amounted to being a death threat.
I don't even think she fully knew what she was doing, and just rode the whole thing on autopilot. Especially since he admitted that "Yes, I know, I'll do my part, but I don't intend to pretend in private that I like it. The job is not "Liking" these arranged marriages after all, and snapping at him for admitting that he didn't like it in what should be private circumstances isn't exactly helping."
He doesn't even really have the option of disappearing and going to ground either, because Karin has both the resources and the will to track him down. That being said, "Karin is a terrible mother" is kind of an ironclad rule of the setting, since she's both absurdly overpowered--and utterly incapable of not being the platonic incarnation of Tsun.
