TOKYO — An outside law firm investigating problems at Nissan, the troubled Japanese automaker, this summer discovered some potentially explosive information.

Hari Nada, a powerful Nissan insider who was behind the ouster last year of Nissan’s chairman, Carlos Ghosn, over compensation issues, had been improperly overpaid himself, the firm found. A second insider involved in the corporate coup was responsible, the firm said, and had briefed Mr. Nada on what he had done.