"Freedom, mother," I hissed out as I stood up, my voice rising in tone. "Freedom of choice is-"
TLDR: Freedom of Choice is a stupid argument and misses the real problems with this particular betrothal, as opposed to other alternate betrothals.
On hindsight, this is a rather silly argument for Henry to make. If he was contemplating other engagement proposals, it is clear he is cognizant of the fact that he lives in a society where the freedom of choice of marriage partners, is rare and highly unlikely within his social class. Hence, his issue is more likely the specific choice made for him, with factors (that Karin can respect and do consider relevant) that are much less present, if outright non-existent with alternate choices.
I don't think it's so much "Freedom of Choice" that is at the root of Henry's protest, and it was an unfortunate conversation ender. Quite obviously, the age factor plays a role in his opposition. But I think the true heart of the matter, is that unlike marrying Henrietta, marrying Isabella is a massive gamble that plunges Henry right into the thick of a "Game of Thrones" esque mess. A gamble with his life as collateral, an entanglement of the entire family into the chaos that is Gallian politics. At least, he could probably justify to himself a marriage to Henrietta - by being a Valleire, he is doomed to be entangled into the internal politics of the Kingdom he was born in, and might be better able to avert or mitigate many of the problems of canon
On the other hand, marrying Isabella, means marrying a probable Lunatic, with a Father-in Law who is basically an even crazier mad-man. At least Henrietta is sane, earnest, and inclined to be just, and Tristian is broadly familiar place where his family's power is easily accessible, and where Henry can fairly easily build and maintain a power-base, as he seems to be doing with the nobles of his generation. Plus, it gives Henry the means to ensure that Void Artifacts get into the hands of his sister as soon as possible.
None of these favorable factors exist with regards to Gallia. In Gallia, Henry is a foreigner with no power base. Setting age aside, he is betrothed to a girl who has quite abit of her father's madness in her (though some of it is due to upbringing), even if Henry might be able to do something to head that off if his cards are played correctly (but this is a big if). As a foreigner whose family likely does not have previous ties to high Gallian aristocracy in recent generations, acquiring a Gallian base of supporters will be exceedingly difficult. Worst still, his father-in law to be is utterly insane. In other words, this is either an extremely risky gamble for peace that might well fail catastrophically with him (irregardless of what he can plausibly do) at the center of the fallout, or an outright suicide mission that his parents have convinced themselves can work because they don't have the S.I knowledge of Joseph's true nature; nor of the extent of Elven interference in the Gallian court.
If he had truly thought through the various engagements, I have a suspicion he has, in some fashion understood the implications of a previously hypothetical marriage to Princess Isabella somewhat along the lines highlighted. Freedom of Choice then is a singularly bad argument (that Henry a huge mistake in even making), because it implies the issue here is not just how crazily risky that specific engagement is for Henry's sanity and life (as oppose to say, a marriage with Henrietta), but rather, a petulant teenage attachment to the romantic notion of free love that has little place in Halkagenian high politics. I think his reaction would have been quite abit less severe, were it Henrietta, let alone any other Duchess or Countess.
There is an important conversation about risk to be had, that sadly, Henry has foreclosed by doubling down on "Freedom of Choice." Isabella was probably one of the worst, most nightmarish scenario (short of quite possibly, Charlotte) that Henry probably contemplated. Sadly, he knows simply playing his part as an imported prince to seal a marriage alliance will fail very hard, because King Joseph is truly crazy, and the marriage alliance scheme unfortunately does not take this into account, because most of the key actors aren't aware of just how crazy the Crown Prince is. Even were Isabella a saint, do you really want a King crazier that Aerys and Joffery Baretheon combined, as your father in law?
The sad part is, it's going to be difficult for Henry communicate that the marriage alliance strategy will fail due to Prince Joseph being a loony-toon, since at this point of time, there's no way he can justify his SI knowledge. What he could have done, however, was to highlight why he thought that that a marriage to Isabella and Gallia was a very big gamble that could fail catastrophically, by highlighting the risk factors present in Gallian politics that would probably be quite reasonable, for someone who has truly given alot of thought and study to the ramifications of each possible engagement over the past few years to identify.
That could open the door for a change in betrothal plans down the line, if these risks factors indeed materializes and proves to be much more severe than Henry's parents and the Tristian Crown calculated, down the line. That approach also would have probably granted him quite abit more latitude to be an active participant in this business cognizant of what is at stake and how it can fail horribly via factors likely beyond his control, rather than being a child that has to be forced into it and deprived of agency for the good of the continent. Which is what we are seeing here.
Alas, Henry has to be in a reasonable state of mind to have pursued this strategy. In his panic however, he choose one of the most sub-optimal and unhelpful strategies of getting out of his engagement with Isabella, or at least creating a legitimate escape route if things go as bad as he fears. Karin is probably assuming this is a case of youthful rebellion that will pass and can be trained out of Henry, but we the readers know that this is a wrong diagnosis that unfortunately, Karin is justified for assuming based on what Henry chooses to emphasize on; with the "you could grow to love her" spiel.
No, the issue is not whether Henry could grow to love Isabella, the issue is that his wife to be is quite mad, and her father is even worse, and he is going off into a Kingdom where treachery is the byword of politics. Karin's line of thinking would be more effective with Henrietta, unless the SI has huge issues with Henrietta as a character in canon, and dislikes her, and is convinced that he can do nothing to affect those issues.
Well, I guess look at the bright side. Charlotte and Louise now become Imoutos....