On the fighting words doctrine itself, you may have missed the note that it relates to “a highly circumscribed, restricted list of insults direct” which are sufficiently extreme as to lead immediately to an affray. This is merely the legal application of the maxim that one shouldn’t issue orders one knows won’t be obeyed.
(Everyone knows that saying something like, for example, “you wear a collar!” is simply a discourteous way to involve someone else in your suicide, and the law acknowledges that it can’t change that.)
On the greater question of freedom of speech: on the one hand, the Imperial governance does not infringe upon the freedom of speech (save in the exceedingly limited case of books which write you back), per Section III Article IV of the Charter; and Imperial society in general maintains a much stronger free speech culture than we do and will not punish you for having or expressing unconventional or unpopular opinions. At most, it won’t invite you to the better sort of parties any more.
However, you are guaranteed freedom of speech, not freedom from all possible consequences of speech, and if you weaponize your words - which is essentially what an insult is - then you are expected to understand that people can and will fight back.
What happens if someone wishes to express their belief about some with a statement like “your mother was a pig-fucking whore who gave birth to a chicken-fucking whore aka you.” or maybe not those exact words but words to a similar degree of contempt and disrespect, who honestly believes those words, and does not consent to getting punched in the face?
Then you’re someone who doesn’t know how the game is played. If you have truth on your side, use it. If you don’t have truth, use subtlety. And if you don’t have either, for the love of gods, shut the hell up. Practice the gentle art of the cut, that socially acceptable form of hostility.
You just violated propriety and social harmony in a big way, are already one step behind for discourteousness, crudity, and lack of self-possession, and handed whoever you just insulted a hand-written invitation to avenge themselves upon you in a manner that will probably lead to them gaining reputational assets. This was, not to put too fine a point on it, a Bad Move.
(And for goodness sake, don’t start bringing people’s relatives into your insults. Doing that grossly multiplies the number of people who have sustainable causes of action against you. Your target’s grandmother doesn’t play.)
But you don’t get punched in the face. This is a civilized society.
You are told that “I cannot imagine what I have done to deserve this abuse, but my second and my barrister are at your disposal, sir”.
You are now dangling from the tines of a most uncomfortable Morton’s fork. Your options are the following:
(a) Accept the duel, and risk being gutted like a fish (literally);
(b) Accept the lawsuit - not that you have a whole lot of choice in the matter if you refuse the duel - and risk being gutted like a fish (metaphorically);
(c) Make your very best sincere formal public apology to your target, to witnesses, and to society in general, and accept the fish-guttingly painful social costs of your fuck-up;
(d) Flee into exile, knowing that Society will be using you as an instructive example of what not to do for as long as memory remains; or
(e) Endure a life of shame, disgrace, and misery as someone with no place in polite society any more, which is particularly awkward considering that the Empire doesn’t have an impolite society. Being a daryteir has rules, and you can’t break them without consequences. This is your personal hell now. Climbing back out of it will not be easy.
tl;dr If you thought the Curia was merciless, it’s got nothing on the Way.