Any of you guys has ever sent such letter to ask abou the wt beliefs? Any suggestions?
Seems like you have realistic expectations on what the response from the WT will be regarding any of their teachings, particularly the one that has come back to bite them in their asses the most.
My suggestion is to send the letter. I think you are right that they are not going to congratulate you or say that they care about you, or that they will admit to an error or lying. However, something else that the letter may do is make you feel good about doing that.
You have a voice, you are entitled to your own opinion and to your own freedom of thought. As a way of reclaiming that right that they try to take from you, I think the letter could be a good thing in that sense. Other than that, I think you are right, they are not going to give you anything good in return.