If some utterances by some people are likely to reduce the freedom of speech of some other people; either because these other people become fearful and censor themselves or even stay quiet (attacked or not attacked), or even killed because of those utterances - then one can argue that outlawing these utterances makes for a freer speech in sum.
Sure, but that's a distinct approach.

And in that case, even if you escape the pitfalls of justification through appeal to some principle, you become mixed in the construction of evidence.

After all, how can a bare communicative event ever reduce the freedom of expression of others (or some other freedom/right of choice) unless in some performative sense, as in ordering the imprisonment of some individual - as in the very sort of abstract thing being attempted here.

IMO if a regime must restrict freedom of expression in any systematic way to achieve or maintain some objective, then it has already failed in it.