You go girl! I think you should send the letter. But, I agree with Dissonant15. Leave the asking for a divorce twice out of it. You are asking for space to heal the marriage, and by saying you have asked for a divorce it sounds as though you have given up.
You are "one flesh" with your husband, and if they are bossing him about behind your back then they are disrespecting both of you. I cannot see any reason why the eldubs would bother you about it, and they may even reluctantly agree with you. Just in case, keep a copy of your letter for your own personal records.
In the past I have had to send letters to JW family while still "in". These people have no concept of boundaries at all. Set some.