Need legal help

I'm employee of an IT company where one of the two partners walked out of the company and asked me to join his new company. I was asked to resign from my existing company and digitally sign the soft copy of the Appointment Letter & NDA of the new company, which I did. Now the owner of my existing company says whatever has been done by me and other employees by joining the new company is not legal because of the following three reasons: 1) The date that I was asked to put in my resignation letter was a previous false date (20th Dec 2020), just to show that I served two months of notice period. 2) The resignation letter was accepted (without any date on it) by the individual who no longer serves the designation of HR Manager. She was earlier the HR Manager. 3) No relieving letter on company letterhead was provided to me. I'm right now not relieved from my previous company and they've put the date of joining in the new company's Appointment Letter as 15th Feb 2021. I'm still with the existing company and there is a bitter feud going on in public domain between the two partners. Will I be in legal trouble if I stay with my current company or join the new company? Can you please guide what is legally the right thing to do? Should I be with my existing company or do I have to join the new company? If I don't join the new company, will they prosecute me as I've signed their NDA. Please advise.