Clarification on procedure to file defamation case against company

Hi, I was working in a startup which is US based. I recently got terminated in probation for “poor performance” of which no email trail is there with me. They did not listen to my explanations and just terminated. Fine. I was in probation so they need not provide even reasons. But when I asked for my full and final settlement, they accused me that I was working in some other company simultaneously and my colleagues had seen that in chat room. I was never working for some other company while working for them. They threatened me to remove review from Glassdoor.They provided me with full and final after I told them that I would go in court of law but can I sue them for defamation and mental harassment? They have many other flaws. They were never incorporated in India while it was mentioned in offer letter that they would be incorporated in January 2018. They had put me on a contract position with a consultancy firm and it is the director of the consultancy firm who accused me over emails