Spoiler or no, you all knew this was coming
After 3 fiancées, acting like he cant have anymore is fucking ridiculous.
Acting like he's morally or emotionally opposed to more perhaps. But if we speak in terms of practicality, isn't it fair to say that adding more ad infinitum would be a problem?
They are fiancees, not concubines, and all will be of royal status once they marry him, if they aren't already. That means they must retain their own noble dignity, personal lands and property, and have housing, trappings, status and authority fitting of their position as wives. Meanwhile Cain also must balance and entertain an ever increasing web of binding political alliances, to which even if he is not opposed, the other stakeholders might. At the very least, he has to ask the other 3 first (actually 5

). It's not like he said no...just not yet.
About the rock...I think its bigger than I imagined from the novel. BUT, since I did this napkin math long ago when I first read this, let me summarize and say in advance to any criticism.....yes it's really unrealistic. But in a 'some details are a bit off' kind of unrealistic rather than "no way, this is just total looney toons shit". I can't remember what numbers I ran but with slightly optimistic figures for the size of the asteroid (and island), the distance from the planet, and the (most important) assumption that the asteroid actually just appears out of thin air and with no starting velocity...conservatively the asteroid would hit with an impact somewhere in the range of 2~10 megatons.
Which is huge but not that bad, compared to some of the stuff we've used to nuke our own planet with, or some of the largest volcanic eruptions known. More to the point, it's survivable by the folks watching...if you assume the island is somewhat further away than it looks and Cain also blocked any heat blast. If compared to an earthquake, the effects would be well below the range of magnitude 9, meaning less than something like the 2011 Tohoku earthquake. As mentioned, the most crucial aspect is assuming the rock appeared with no velocity in mid-air, instead of being drawn down from space, because most of the kinetic energy comes from an asteroid falling into Earth's gravity well. Just assume some conceptual magic whatever...
Just a for fun thing.