You've always been a cool cat! Now I don't know your mom, but I believe she will read what you say, even if it hurts her. It may be different since your dad isn't a Witness and I assume you two will still have a good relationship. It might be weird for her to be on the outside looking in at that so she might not shun you like other parents do. Bottom line, she needs to know you're an adult and capable of making your own choices, and this letter will definitely show her that. Great idea.