I can't see the value in getting DA'd or DF'd in your case. I know it would shake your father's world a little, but it could end up being a losing choice should he elect to 'get serious' about the society's rules in this matter.
Sounds like to me your dad is a great guy, maybe just going along with the WT rules due to his wife and step-kids [if I got that right]. If you got Df'd he may face pressure from his family or someone in the congregation who makes it hard for him to treat you well and stay a witness.
I vote; if it ain't broke, don't try and fix it!
Jeff