I would make a very pointed phone call to the Elders who said that, and tell them that since they confirm that you are a big girl now, that it is NONE of their effing business if you and your mother associate together. You both quit their stupid asinine micromanagment bull$h!t cult and you do not answer to them. And then remind them againwhat they said above, and tell them that YES you are a BIG GIRL now and don't want them bitching about your associations with your mother who is no longer part of their stupid asinine micromanagment bull$h!t cult and no longer answers to them either. The fact that your father and mother are still together and still go places together is not a matter for the Elders to interfere with. It's NONE of their business. You can threaten them with a harassment lawsuit too.
If they continue to interject themselves in between your mother and father, have your mother remind them that they are still married in the eyes of God and that Matthew 19:6 says, "Therefore what God has joined together, let not man separate." -- New King James Version
But do this in a mild mannered way, not in the heat of anger. As much as I admire Scully, and yes she is right too, I think she might be missing the big picture here as to which method of approach to employ. You do not want to loose your cool in this situation.
You want to contrast yourselves to the elders in your father's eyes as much as possible. This will help him see more clearly what they really are. Again, take the high road. You want to interject doubt by introducing key thoughts and questions sparingly. Don't use a direct attack; cult conditioning causes members to become defensive of direct attack. The subtle approach is much more difficult, but it is what has worked in the past for people like Amazing1914.