The inside story of how Elon Musk won Twitter—and could still lose it

For a man seeking the biggest margin loan in history, Elon Musk was surprisingly calm. 

It was Easter weekend, and Musk was deploying a charm offensive to court lenders. “He was disarmingly normal,” says a source who participated in a 45-minute video call with the Tesla CEO, who was piecing together the funding for his just-announced $44 billion bid for Twitter. “A lot of tech entrepreneurs will say, ‘If you can’t keep up with me, you’re wasting my time. Elon had strong opinions, but he wasn’t arrogant.’”

But the world’s richest man was...