I worked in death claims for over a decade and during that time I would have seen literally thousands of death certificates. I used to see "old age" written as the cause of death fairly regularly for the very elderly, it's not suspicious or uncommon.
The fact that nothing else was filled in on the death certificate would seem to put paid to the rumours that she had advanced bone cancer. That would have been listed on the death cert as an underlying cause if that were the case.
When the cause of death is given as old age, it always sounds rather a peaceful end to me.