I, too, simply faded and didn't bother to formally disassociate. And they didn't bother to disfellowship me, even though they had legitimate grounds to do so several years ago. I had moved far away and hadn't claimed to be a JW for many years, so as the holy writ from their religious overlords in NY suggested, they just held the matter in abeyance.
Besides not wanting to play their game by feeling like I owed them a formal statement, I thought that by not being formally DA or DF, it would leave the way open for some JW family to maintain a normal relationship. Some did and others have shunned me anyway. Some treat me as they always have and others act like I have a price on my head. Go figure.
My dad was a long-time elder, but he never shunned me or asked me to stop calling or visiting him, even after he learned I was "living in sin" (at the time). He yearned for me to "throw myself on the mercy of the brothers and return to Jehovah," but I explained it would be of no value if I didn't really mean it. It would be pointless and hypocritical. I wouldn't be fooling anybody. Reluctantly, he finally accepted things as they were and we moved on. Most other family members followed his lead until he passed away. Then the self-righteous and judgmental ones made a big show of cutting me off from anything family related, not even informing me of major events like deaths, marriages, and births. Their loss.
Anyway, I think you are handling this situation admirably. Good on you for helping your dad despite the fact that it probably wouldn't be reciprocated if the situation was reversed. You're setting a better example of "Godly love" than most JWs do, despite all their bloviating and self-promotion.