A little over two years ago, William A. Ackman, one of Wall Street’s brashest and most self-assured hedge fund managers, was on top of the world. A billionaire before he hit 50, he was generating double-digit gains for his investors and raking in hundreds of millions in fees for his firm and himself. Hailed as a master investor, he clinched his highflier status in the fall of 2014 by paying $90 million with some friends to buy the penthouse at One57, a 13,500-square-foot aerie in Midtown Manhattan overlooking Central Park. He didn’t plan to live there — it was an...