Mystery Monday: How Can I Find the Elusive Nellie Block?

Great aunt Nellie Block, late 1940s

Nellie Block (abt 1878-1950) is my elusive great aunt, the older sister of my paternal grandpa, Isaac Burk (1882-1943).

The first time I spotted Nellie was in Isaac's 1904 border crossing from Canada to US, when he said he was going "to sister Nellie Block, 1956 3rd Ave., corner 107th St." The address was familiar, because Isaac's future bride and her family lived in that apartment building!

In the 1905 NY Census, Nellie (a furmaker) is living as a boarder with a family on Henry Street. She's still single, and boarding with a different family on Henry Street in the 1910 US Census (occ: operator, furs).

The paper trail nearly ends there for Nellie. So far, I haven't found her in the 1915 NY census, 1920 US census, 1925 NY census, 1930 US census, or 1940 US census.

I know Nellie received an invitation to a UK cousin's wedding in 1934, because it was passed down in the family. Alas, no envelope with address. Did she go? No one knows.

Nellie is wearing a corsage and a smile at my parent's wedding in 1946. That's how I can date the photo at top, because Nellie looked very much the same at the wedding as she does here.

The final record I found for Nellie is her death notice from the New York Times, paid for by the family. It states: "Block--Nellie, devoted sister of Abraham Birk, Meyer Berg, Max Birk, Jennie Salkowitz, and the late Isidore [sic] Birk. Services Sun, 12:30 pm, Gutterman's, Bway at 66 St."

Nellie Block died on Christmas Eve, 1950. I haven't yet found her burial place, and can't yet get a copy of her death cert from New York (too recent).

Where in the world was Nellie Block hiding between 1910 and 1950? My next steps, part of my Genealogy Go-Over:

• Use Heritage Quest and Family Search, plugging in different spellings of her name to search US and NY Census records. Each site transcribes and indexes a little differently, so I may have some luck with this approach. Will also look for naturalization papers, if any.
• Do a more thorough search of Social Security applications. If she was working, and remained single, surely she filed for retirement benefits, right? 
• Check NY marriage records, just in case she married at some point. By 1934, however, when she received the wedding invitation, her name was still Block and she was about 56 years old. I suspect she didn't ever marry, since her death notice is "Block."
• Recheck Find a Grave (so far, I haven't found her there) and all the NY/NJ cemeteries where my NY-area paternal ancestors were buried. My really quick first check was unsuccessful, so now I have to do another check to be sure.
• Any other ideas? 

UPDATE: I searched census and naturalization via Family Search, no luck (yet). Also did a search on the easy-to-search 1940 NYC directories on NY Public Library site, borough by borough, but no luck. In addition, I checked Italiangen.org for naturalizations, but no luck. And I redid my Soc Sec search via Ancestry for claims and application, no luck. Darn.