I came across this article from 1971 mentioning numerous heroin overdoses in the neighborhood and how there was a heavy heroin presence in the neighborhood . If you read the article they even mentions the block that Denise lived in as one if them. This brings me back to my theory of accidental overdose in the cemetery.
Narcotics Are Complicating Sunnyside's Problems
Captain Langan said “a lot of the kids in the area are on junk — hard stuff.” Sunny side's drug traffic is concen trated in two areas, on 48th Avenue between 44th and 48th Streets, and in and around an old, pitted park at 43d Street and Greenpoint Avenue.