The supposed buyer had said his name was Karl McNamara. With some computer sleuthing, Mr. Singleton discovered that Karl McNamara’s Facebook profile had used a photograph of Patrick J. McNamara, a New Jersey lawyer who specializes in land-use issues. (Patrick McNamara said in a telephone interview that he knew nothing about the mansion or the historical society and did not know a Karl McNamara.)

Mr. Singleton recounted the hopeful exchanges with Karl. He said Karl had first talked about backing Friends of the Steinway Mansion in bidding to buy the place. Shortly before he vanished, Mr. Singleton said, Karl had taken a different tack, explaining that he and his wife had decided to buy it themselves.

Karl then emailed that the school would be called the Steinway Conservatory and would be “dedicated to providing a tuition-free, world-class musical education” for students from low-income backgrounds. The memo had bullet points about an emphasis on “ethical behavior in all aspects of personal and professional life” and on “establishing a community that values integrity in all relationships.”

Mr. Singleton said he never met Karl except by email. “I was upfront on my concerns that their Facebook page was obviously manufactured,” Mr. Singleton recalled. He said Karl was reassuring, though when Mr. Singleton pressed for a face-to-face meeting or even a telephone conversation before the Sept. 21 presentation, Karl sidestepped the requests.

The hoax played out as construction crews built a retaining wall and began work on new light-industrial buildings in what was left of the mansion’s front lawn. That is why Mr. Singleton now refers to the hoax as a sideshow. To him, the main event is the mansion itself, even with the changes to its surroundings.

The mansion was built when this corner of Queens was second-home country for moneyed Manhattanites. It is “the only surviving example in Queens of an aristocratic Victorian country home,” the historian Vincent F. Seyfried wrote in his book “300 Years of Long Island City, 1630-1930.” The writer and tour guide Sergey Kadinsky called it “a patch of green amid industry” and added that it “could be reminiscent of actor Chevy Chase’s 1991 visit to Valkenvania” in the comedy “Nothing but Trouble.”