For the definitive answer you need to speak to Tom Bird when you get a chance. Tom was the driving force in getting the city and Park to purchase the U Thant estate and make it a park adjacent to Seton Park. I was on the Arms Around the Park committee and group that Tom started to save the strip of land from developers. My understanding is the U Thant house was located on this parcel of land. The entrance is noted by the two large stone columns you see on Palisades north of 232nd St.
Our political leaders at the time waffled (what else) on whether to support the land becoming a park or letting the developer build on it. The Boro Pres. then Freddy Ferrer, was for it before he was against it then was for it and it finally became part of our parks. Why Wallenberg Forrest I don’t remember. No one I know called it or calls it Wallenberg Forest. That is just another stupid name Parks Comm. Henry Stern decided on. Much like he gave silly names to NYC Park workers like they were horse or servants who had to wear the idiot name tag he made up.
Why not U Thant? He lived there and was Sec General of the U.N. but had no real connection to Riverdale or the Bronx.
Could have been worse if it Kurt Waldheim who lived here. Imagine having the park named for him.
Why Raoul Wallenberg? Again best to speak with Tom Bird. What I remember that about this time there were news articles that Wallenberg might still be alive in Russia. Russians stated he died soon after the war after he was imprisoned by Russia. Also some belongings were returned of his and there was articles of sightings of him in Russian prisons years after the Russians said he was dead. Many places around the world were naming sites for him and believe this is how Raoul Wallenberg’s name was used for the park.
Again all credit for saving the property as a City park goes to the effort of Tom Bird who was able to get our politicians, and community rallying around the idea to save the land from developers.