First UA, very sorry for your loss - he sounds like a wonderful man and someone who even when he is gone will help guide you through your life with what he has taught you.
As for the funeral, it is a celebration of his life and you should go and celebrate him for what he has meant to you. We went through this last year with my wife's grandmother and the squabbles that went with all the planning - it is not fun. But, all the same, rather than fight about it, go and celebrate him, what he taught you, what he meant to you and your mom.
Possibly, afterwards, why not arrange a private memorial service for him as you and your mom wish to plan. Maybe your church could help you with this, nothing elaborate, just another celebration of his life the way you and your mom wish to do it.
And again, very sorry for your loss.