Oh Julia! I really feel for you. You must feel kind of exposed emotionally by her response, huh? Your letter clearly shows the high regard with which you held your friend. Your saw her as being different - you did not want to un-friend her. She still matters so much to you. And that's the rub, isn't it? If she had treated you as badly as the other JWs, it might be enough just to un-friend her. But at a certain level, she matters to you and you feel for all she has been through in her JW-life. Yet, she doesn't respond to the underlying message in your letter that her friendship means so much to you. It's a sad case of unrequited friendship.
One of the risks of reaching out to others in the organization we consider to be 'friends' is they won't reply as friends but they self-protectively put on the 'witness' hat. They may well be really nice and caring people at one level, but at another, the switch is off. It's even harder for you now that you tried reaching out to her. I guess you could say "at least" she did not completely ignore your letter and "at least" she was still prepared to try to reach out to you with her beliefs. It just wasn't what you were expecting, though, right?
It's probably a bit too late now, but it may have been wisely self-protective of you to have had a Plan B, so that when you got the 'witness' hat reply, you'd be clearer about what you need to now do to look after yourself. I shouldn't imagine you are too keen to send off another letter just yet until you weigh up the pros and cons.