I'm so sorry...I feel exhaust reading your post...You are working so hard and have so much to handle. I agree with Ann in regards to your grandchildren and also about calling the court. As for a letter to your daughter; if it helps you to write it all out, then go for it...but maybe just rip it up when you are done. Because unfortunatley, if she is in active addiction, her first love is always the drugs, sad to say, and anything you say to try to convince her otherwise will fall on deaf ears.
I'm sending lots of hugs and many prayers for you and yours. Please try to spend any spare moment you have focusing on yourself. She won't get it until she is ready and there is no use having you sick as well.