Tesla CEO Elon Musk’s attacks against “sanctimonious journalists” and “divisive” unions have captured the public’s attention in recent weeks, but Musk has bigger problems on his hands: His automaker has struggled to deliver on the promise of a mass-market fully electric car, the Model 3 sedan. When Musk delivered the first Model 3s last year, he jokingly warned customers and employees that the firm would be entering “production hell.” But according to several former Tesla insiders, it’s not a joke. Cars built by hand, fires that turned a paint sprayer into a flamethrower, rampant quality problems, and a bloated workforce...