Golan didn't choose the hive mind over the Seldon plan. He chose the hive mind because in the short term (the 2nd 500 years of Seldon's plan) it would maintain the status quo.Yeah, what you're thinking of is The Mule - that's when the Foundation went to see what the Vault said and it was completely off. Then the 2nd Foundation fixed that by brainwashing The Mule, and making the Foundation think they destroyed the 2nd Foundation. Psychohistory and the Seldon plan were not fake, the encyclopaedia was fake. Seldon and the 2nd foundation did believe in the plan and the 2nd foundations job was to monitor it and make course corrections to it as needed.
Then in the 80's what's his name chose the hive mind over the Seldon plan using the logic that if Psychohistory actually worked, he shouldn't have had the option to make that choice in the first place, as psychohistory relies on the fundamental truth that no individual can affect the future, it is strictly determined by mass actions. By merely being given the option to choose, it showed psychohistory was inherently flawed.
Also, Pshychoshistory doesn't say no one individual can affect the future, it is that it can't predict the actions of individuals. That is why the Second Foundation exists, to correct for individuals and to always keep an eye out for a better solution. That is one reason why they orchestrated the crisis so that they were left with only a single path forward. And if you pay attention to the stories, very often an individual made the crucial decision that solved the crisis.