I am also research counties in North Carolina, but unfortunately, I am not researching Wake county and don't know what records maybe available to you. I notice that you stated that your "ancestors seemed to have serial marriages". Fortunately, I find that an advantage in my family research. Some of the earlier records did not include the parents name or the information was not legible. Once some of my ancestors married their second or even third wife, I got a chance to possibly retrieve the same information again about their parents. In one case, I thought that my Great, Great Grandmother may have been died. Later I found that her son married and the mother's last name was correctly noted with her current marriage that I was not even aware of.

I am not complete understanding all of the names and the history that you had mention in your posting. But if I am understanding it correctly, you are researching an Easter McMannen. I've notice her with her son Nathan McMannen in the 1900 and 1910 US Federal census. You mention that she was born approximately 1835 and possibly died before 1920. You also mention a death certificate of a Easter Preddy in Wake County, not too far from where your Great, Great Grand mother spent most of her life. A closer look at the death certificate shows that this "Easter Priddy" appears to have been born around 1874 and married to a William Priddy. This Easter Priddy died Dec 22 1919 in Wake county NC. A look at the 1920 US Federal census for Wake Forest, Wake county, North Carolina SD 4, ED 148, sheet # 3A, Lines 33-38, shows a William Priddy as a widow and in his household his children. Looking back at the 1910 US Federal
census for Wake county, North Carolina SD 4, ED 132, sheet # 123, Lines 69-75, there is listed a William
Purtty with his wife Esta and their four children, also a boarder. I may be wrong but from the information that you mention and the census information that I am finding, it does not appear to be the death certificate of your Great, Great Grandmother but appears to be this person documented in the 1910 and 1920 census born approximately 1874.

It is great to know that you will be able to travel to NC and research the Wake county records. I would be curious to know if there were city directories of the area that you are research. Perhaps it may have your Great, Great Grandmother listed (or it may show only the head of the household). Do you have information on all of Easter McMannen's children. You may consider researching Nathan McMannen and the rest of his siblings to find out where they were buried. Perhaps their is a family plot or only one cemetery available to blacks during that timeframe that might help you discover where Easter McMannen was buried and when. Since Easter McMannen appears to have lived with her son, Nathan McMannen until her death. I would consider checking on the cemetery where his family was buried first.

Sorry that I could not have been more help to you.
Good Luck!
Allen McClain