The death certificate of Mary King O'Hara: examining the document that changed everything

My 2x great-grandmother, Mary King, died on 5 November 1949 at the White Nursing Home in Brooklyn. This seems to have been a type of long-term care facility, and yet her "usual residence" is given as 505 Sixth Street, the Brooklyn row house where she had lived for many years. (A bit of newspaper searching yielded very little information about the facility, besides the fact that it was advertised as "Cheerful rooms, home atmosphere, excellent food and care. Licensed." That was a classified ad that ran frequently, maybe daily, in the Brooklyn Daily Eagle during the 1940s.) The Department of Health won't release the cause of death to anyone who can't prove that they have a reason to need it and the right to have it, so I cannot glean any information about her last days from her final illness.

Death Certificate of Mary King O'Hara. 5 November 1949. New York City Department of Health and Mental Hygiene. 

The informant on this death certificate is my great-grandfather, Mary's son John J. O'Hara. He lived in the same building; he had rented an apartment from his parents, the owners, until the building was sold in 1946, and now both mother and son were presumably tenants of some other landlord. The O'Hara family had spent several years in Ireland when John was a boy, and it seems safe to say he would have met his parents' Irish relatives. He could have known the grandparents he named on this certificate. All in all, John is not the least reliable informant a death certificate could have.

Which is why it really gums up the works when the grandparents he names are not the ones I expected.

One piece of information, though, makes me wonder whether John was a truly reliable source, or whether he might instead have been confusing dates. Or was it that he had an excellent reason to get them right? Mary's date of birth is given as 3 December 1875. Her husband, also named John, had died 3 years earlier on 3 December 1946. Did John Jr. provide a date that was familiar for the wrong reason, giving his father's date of death rather than his mother's date of birth? Or was it a date he was sure to get right, forever in his mind after having lost his father on his mother's birthday just a few years ago?

Beyond the issue of her parents' names, the only slightly surprising piece of information on here is Mary's middle name, which I hadn't known, although she was routinely "Mary E." on records.