I'm so sorry, but there is no record of the death of Eleanor Genevieve Fosdick Peak/Pieke Cassebeer. It is possible that she married again, although I found no online record of her marriage after the death of Henry Cassebeer in 1941. Many, many states do not put marriage records online, so it would not necessarily be available.
I did see the 1945 SS claim; also the note dated 19 Jan 1978 that states that her name was listed as Eleanor G. Fosdick.
What you can try is writing to Social Security to see if there is a death record for her social security number. It's even possible that she changed her name to something else to avoid publicity about the Titanic in her later years. While she might have changed her name to "***** *****" the social security number would have followed her. This isn't something that can be done online. She isn't listed in any of the online SS databases.
I've tried searching for Eleanors born 29 Nov 1875. There was an Elinor, but she isn't your Eleanor Fosdick.
I'm so sorry there isn't any information available, in spite of hours of looking.