Another thing we've overlooked!
How would he leave properties to family friends and nieces if he didn't know he would die??
He would only plan a will that meticulous if he had the plan to fake his death probably.
And, Remember Jacky Kennedy remarried Onnasis , a very very wealthy man from Greece, they have their own island and massive wealth. Jacky was still married to him when she died, if he didn't die first.
However, as a mother she would have known John John faked his death, either immediate or after. She lived mostly in Greece at the time of her new life with husband. He would have plenty of means to hide John John to be with his mother even till her death.
I never saw her as a fragile mourning woman even after John Jr's crash. I think some digging into her may give clues too