What all actions I can take if my org, manager or hr, rejects my resignation or buyout?

Rejects your resignation? I'm sorry I didn't understand the title?

Your resignation email and last date means it's final a company cannot say anything if u are leaving on ur notice period end date . A company cannot force u to stay or work

This is extreme case doesn't happen, nobody can force you to work, do your part, drop mails to assign a person for KT, give KT as much as you can, have everything on mail, take confirmation from a person that KT is given, Then don't show up once your notice period ends

You are overthinking. What folks say here is correct.

There's no such thing legally. As soon as you tender your resignation, you're only bound to work until your notice period or a last day of YOUR choice with buyout as provisioned in your employment contract.

Buyout is legally binding on both parties and a company cannot reject that.

Usually, companies do these time wasting time tactics and make it difficult for you, until they find another person.