You and the fiance can arrange the use of her Kingdom Hall (or whichever one) before you move, before any letter of reference is out there. Do it now. Set a date and location and get permission right now- this week. The BOE of that hall will check on you with the elder who loves your free or discounted mechanical services and give you the barely-enough-of-a-good-reference that you need. It will already be arranged and done.
Then if you move, they won't bother rechecking. Even if you arrange it for your new hall, the elders will not want to back off after getting a good-enough reference for having a wedding when they compare it to the letter they receive. Just make sure you continue to stay "active" somehow.
If you cannot do all that, then tell mother that you are sorry she feels the way she feels and you hope her absence at your wedding would not mean her absence from your life.