OK, here's my shot at a reply (draft) so tell me what you think. I'm reconciled to the outcome that she may break off our friendship over religious differences, but I try here to reach her genuine self.
Hi S***** I appreciate your kindness, and I can assure you my spiritual journey is continuing and progressing. Like you said yourself, in 1 Cor 11:3, it is ultimately Christ and God who are our heads. I come to Jehovah through the Son, who said, "I am the Way, the Truth, and the Life." Therefore my relationship with Jehovah is an intensely personal thing. It exists irrespective of cold hearted, legalistic, deliberately cruel people who live to a double standard. As you asked, I have discussed your kind offer with [husband], and we will politely decline it. We live in accord with the Bible's guidelines, and although many treat us as though we are disfellowshipped, we are not and have not contravened the Bible's moral code or other commands in any way. However, religion itself tires us. It causes me great mental distress, and that in turn hurts [husband]. God is truly loving, and Jesus has taught us that love and mercy are the essential components of true Christianity. Therefore we leave ourselves in Jehovah's care, and continue on life's journey in spirit and truth.
And that's as far as I've got so far. I use the name Jehovah because it's a concept she understands, because not using it would be more noticeable than using it, even though I don't believe in that name. I'm pretty much agnostic at the moment, but live by Christian values.