In all my years I have only worked with one cheater..........I was working 3 man mechanics in a playoff game. My regular partner and I are the base umpires working with an umpire who is clearly pinching one pitcher......the strike zone is wide for one and narrrow for another.....it did not take long for my partner to come to me in between innings to confirm what we both were seeing.
We called time and walked in and the conversation was short....It was what I would call a "come to Jesus" meeting.... he was cheating and if he missed another pitch he would be explaining to the world why we base umpires just left the field, because we dont work with cheaters.... He never missed another pitch again that day and we never worked another game with him again.....(and never will)....
The puzzling things from your post to me are:
The TD saw this and never said anything?
The Umpire was not calling , how did you describe it? "dead gutted middle pitches" .....
Tournament directors usually arent shy about interjecting themselves into the play.......this weekend, our TD told us to call more strikes as he didnt want any long games.....and that we were missing technical balks....both of which we did not call more of.....
Then there is the Umpire who is pinching a pitcher...I would say they would rarely call a ball on an obvious strike.....especially with witnesses right behind the screen.....the zone just seems to shrink from the corners in, as in my example above.....not denying what you say is correct, but it just sounds odd to me.....
I never called a level in my area that my son was currently playing at......but once he moved up, I did. I choose not to do my local school districts teams.....Sometimes ethics are more about appearances than hard fact......
If you think it would make you feel better, then write the letter. It certainly wont hurt anything. Yes it may come off after a month as being just sour grapes, but you will alert the TD staff of the appearances of unethical behavior and they may alter their stance in the future....