A Chinese firm helping former president Donald Trump take his new media company public has been the target of investigations by federal securities regulators, who say the firm misrepresented shell companies with no products and few employees as ambitious, growing enterprises, documents and interviews show. Arc Capital, an investment advisory firm based in Shanghai, has repeatedly helped create or finance companies with little or no revenue, no customers and office locations that point to P.O. boxes, according to a Washington Post review of regulatory and court filings. One claimed to be developing autonomous drone software despite having no employees; another...