Sorry for the crap you and your family are going through, Ellie.
I respect why you might want to do the DA. For my wife and I it helped with closure, moving on, and finally an attempt to clarify that this was totally our decision to leave, not theirs. Not a gradual "weakening" and running away that in their eyes may look like we were too ashamed to face them, "causing" a DF on their part. I realize that apathy (not "playing by their rules" as poztate noted) probably represents an emotionally stronger reaction and/or being "over it", but this felt better for us. Possibly for similar reasons as yours, since we also had tons of family and friends inside and yes, it did matter a smidgen what they thought. At least the basics. If rumours etc continue to fly after that, so be it. At least you've made your statement.
Since you expressed some interest in "maintain(ing) certain family relationships", perhaps another reason for the DA is to keep these options open - if not now, in the future? If so, perhaps sending the formal DA to the congregation that describes the real "truth" of your situation, and including copies of this DA in personal letters to those you still care about (the poor fools) can help this. In those letters you may extend a hand, if they want it. For example, while you are firm in your decision, that doesn't alter your love for them. And suggest they let you know to what degree, if any, they are able to sustain the relationship, so that unspoken assumptions aren't made blah blah blah. Maybe this only applies to a few, or maybe none at all and it's time to burn all bridges. But if this is a consideration for you, then if all they hear is a DF announcement and no further contact from you they may think you're not interested. If no one makes the first move you may all go to your graves with no further contact (sorry, don't mean to sound morbid). Maybe that's the best for you anyway (in some cases no relationship is better than a painful relationship with a loved one). But as others have said you have to do what you feel is right for you. As with many things, no one course is best for everyone.
Your initial question asked what harm might come from this. I don't know, maybe you'll look needy or apologetic, etc which you may feel is worse than "no comment"? But you might cover that by stating your reason for the letter at the beginning (e.g. to dispel any false rumours). Can't think of anything drastic. As far as I know they don't have an underground extermination squad who'll take care of you, though (not 'til Armageddon, right? harhar). Or else they just haven't got around to us yet!
So if you do go through with it, don't worry about the reactions. You're only being honest. Just get it off your chest and be done with it. You go girl!!
