I agree with Hobie in that a bar complaint in this instance will go nowhere and cause you to use your precious time filing crap with them instead of addressing your problem. If you can prove you truly have a legal easement, I would ignore the letter and stay the course (not legal advice).
If Mr. Wacko sues YOU, then you can answer it.
It sounds to me here like your neighbor paid his (relative?) PI lawyer $50 to write a “mean” letter to you based on what he was told without doing any research (since if you truly have a legal easement and you did NOT try to remove his fence or threaten him, he has no cause of action). Believe me, stranger things have happened.
Sometimes I wonder if most of these ambulance chasers even know how to conduct legal research or draft a complaint without checking boxes on a preprinted court form (or have ever even done so since they got out of law school), lol. PI attorneys mostly operate on volume (and contingency) so don’t really spend too much time on any one case unless it is a vehicular homicide, wrongful death or egregious med mal case.
Have you had your 1-2 hr paid consultation with a qualified RE attorney yet?