Reading your recent posts, Tracey, I, too, realize you have an especially close relationship with gd, That must have made it very painful to have the contact cut back, and even worse to have it co altogether. The 5 months must seem like an eternity! Not so for dd and bf perhaps. So it may take a while longer for dd to be ready to reach out to you.
I don't think you should send anything else to dd. If she has blocked your emails or refused to read the one you just sent, she probably isn't going to read any letter you send him. And I agree she might see it as disrespecting her desire to go nc. If you do send a letter, I hope you include an apology in it for your mistakes. That might be key to opening the door.
I'm torn about gd. Otoh (on the one hand), if you send her a card, dd might see that as disrespect, also. Otoh, it worries me that gd cried and expressed the fear that she might never see you again. I hope dd has reassured her that she will see you again, eventually. But just in case she didn't, a card from you might lift gd's spirits, letting her know you're not totally out of her life. That is, if dd gives it to her. It might be worth sending gd one card, just to let her know you're still interested in her, even if dd gets mad....Idk... But if you do, after that, I would let it go.
I get the impression that you're a person who is used to being proactive and that, therefore, it's hard for you to just wait and do nothing, or just apologize and wait. But I think that's what you need to do. I wish you all the strength, courage, and wisdom you need to do it.