Rhiannon, lots of love and best wishes to all of you, I'm glad your dad is at peace now. I wonder if he chose to slip away quietly while you were all out of the room?
Just in case your DS changes his mind - mine was 8 when his grandad died last year, he hardly ever saw him and they were not close but he did come to the funeral and I think it was better for him than not coming. He had a new exercise book in the car on the journey down and wrote something like "81 years is a long time to be alive for one man" so he was obviously giving it some thought.
My father used to love My Fair Lady and we had 2 of the songs played at the funeral, which was quite cheering - especially as "I've Grown Accustomed to Her Face", played at the end, begins with Rex Harrison shouting "Damn! Damn! Damn! Damn!" - which made DS2 sit up a bit!
As far as the post mortem is concerned, they may find something that would be useful for you to know; my mother died of mesothilioma (cancer of the lining of the lung, normally associated with asbestos) but a post mortem revealed that she actually had ovarian cancer, and the lung thing was probably a secondary; her mother also had ovarian cancer and now we have a chance to be monitored for it.
Look after yourself and yours. Hope all goes well. xxx