I am NOT an attorney.
1. My question as to who paid for the cremation etc. had to do with who the funeral home might have seen fit to release the cremains to. My stepfather was cremated about 3+ years ago and I picked up the cremains at the funeral home. I signed for them, but I don't remember what ID, if any, was required.
I mean you can't (?) just walk in and say
"Hi! I'm here to pick up ---- ----- and walk out with an urn. / Just finished reviewing the paperwork. A 4 page document , entitled "Cremation Authorization and Order of Disposition" There is a line on the document that says: Deliver said cremated remains to: and written in --return to wife. To the side is a blank space where I initialed upon pickup. On the flip side of that is my mothers witnessed signature. I don't live in NC but I'm sure a similar form is used there. You need to check with the funeral home and see who directed the remains to be returned to whom. If it wasn't done correctly then the funeral home may have some liability. The info thereon can conceivably help or hurt your case.
#2 As I suspected your mother and father were estranged in his final days.
Immaterial what the reason was. I can envision your father returning to his father. When someone (family included) hurts a child the parent becomes very defensive. Maybe your father told your grandfather to actually take his cremains, maybe ,maybe-who knows now.
#3 "The ashes were to go to my Mother and her 3 sons (me)." Excuse me but you state this like it was a established fact. It's like a heir saying "but I was supposed to get his rifle or harmonica or car". I admit it is customary for the spouse or next of kin to receive the remains but it is not a legal requirement. Some people even state in their will what special disposition of their remains they want.
#4 As I stated earlier your grandfather has physical possession of your father's ashes. Unless he came by them in some underhanded way, IMO the court would lean his way.
#5 By being heavy handed you may cause a further and deeper rift in the extended family. Is there some family member that both sides would approve of that could act as a mediator to solve this without going to court?
#6 Actually I cannot think of any legal grounds either side could use to either retain or gain the ashes. Imagine this, the judge says let's put the lawbooks away and I want to hear a statement from each side as to why they think I should award custody of the ashes to them. Limit your statement to 40 words or less. Then I will rule. Go. What would you say?