Jookbeard
I don't believe the elders would know what your mum's wishes were, as they are only there to protect their religion. If they really cared about your family and your mum's wishes they would not be breaking families up. I don't won't to tell you what to do, but there is nothing wrong with a DF'd person attending a JW funeral.
I am not Df'd but have faded but when my sister in law passed away about a year ago, there were a lot of people that were DF'ed that attended her funeral, they were not sure if they should go but felt better after they did, they did this because they wanted to pay their respect to her has a beautiful human being.
Take care in whatever decision you make xx