So sorry you are in this situation.
This short time available to you should be focused on your father and being with him, and being with your mother to provide strength and comfort.
This is not the time to discuss any "issues" the JWs have, and you need to let them know this, repeatedly if necessary. There will be plenty of time later for you to formulate plans for dealing with elders, etc., but no time left with dad. When the time has passed, you will be able to look back and know that you did all you could for your dad, and that is the important thing at this time. It does not matter what the words are that are said at the gravesite, just the fact that you are there to remember your dad and be a source of support for your family.
Do NOT get "sucked into" a religious discussion of any sort, no matter how hard anyone tries. You will leave frustrated, and will be giving them what they want, no matter how it comes out.
Wishing the best for you at a very difficult time.