I think the letter was great for venting your feelings, however I would recommend ripping it up now. Imo it won't bring about any positive changes for anyone. I totally think meeting with the teacher face-to-face would be good, but only if you can remain calm.
What the teacher did was completely uncalled for and wrong and manipulative. However, I try to be a bit compassionate toward teachers because they often have a lot on their plate. She needs to know the punishment did not fit the crime, but I don't think she needs to have words like "disgust" thrown at her.
My dd had a really rough time in Gr. 1. Gr. 2 on the other hand was wonderful for her. I think the teacher made a HUGE difference because she was much more positive and encouraging imo than the teacher in Gr. 1. However, I have no ill feelings toward the teacher in Gr. 1 because she was doing her personal best. I also made it a point to let the Gr. 2 teacher know at parent-teacher interviews how impressed I was with how she had been helping dd with reading and being so positive in reinforcing good behaviours with dd.
A lot of the time teachers only hear what is wrong. This instance with the mealworms definitely should be brought to her attention as the wrong way to handle a situation, but "I" statements and calmness are more likely to make her think about her actions and hopefully change them for the future, than harsh words will, and I'm guessing since you are pulling your son out this week anyway that your main goal is to try to change her behaviour in the classroom toward all the children, not just your son.
I know I might get tomatoes thrown at me for my comments, but I just try to remember that teachers are only human too and although they should be called out on inappropriate behavior, they also should be treated with kindness. They often have a lot of pressures put upon them. I go on teacher boards sometimes to get ideas and you wouldn't believe how many teachers are stressed out and burnt out by trying to be everything to everyone. Also, just before the holidays it might be harsh to end on such a bad note with a harsh letter.
Now all this said I have never met your son's teacher. She may be someone who is kind but misguided or she might be someone who is snotty and mean-spirited. If it's the latter then maybe she deserves the letter, but my dd's Gr. 1 teacher for example, although missing the mark in some areas I believe, was truly trying to do her best and help the kids in her class learn.
Best wishes to you as you start your hs'ing journey. I started mine with my two children (5 and 8 yrs. old) this past September and it's hard to believe it is already almost the end of December!
Btw, if you ever are interested in meeting up, I am in Windsor, just across the river from you. I'd love to meet up with someone who knows good learning places to go in Michigan and we can go on a group fieldtrip sometime