As I can get from the entire write-up there are 2 issues -
1. legal action from CA
2. relieving letter
For legal action, as I can understand that there was a mutual understanding between the organization and CA farm so I don't think that CA will do any legal case against you. I am here bit confused why CA is not accepting written resignation letter if he had agreed verbally and mutually discussed with NAPL.
In this situation there is nothing much to do but you can send an email of your resignation which should contain the following points -
1. date of your actual resignation
2. details of verbal discussion
3. Reference of the discussion with date between CA and NAPL
4. any other point, if discussed.
This email will help you if there is any legal done from CA.
For relieving letter, as there was a mutual understanding between CA and NAPL so they might not ask you for the relieving letter but if you want to join some other organization in future they will definitely ask for for it. At that point of time you can use this email and tell them the entire story.
Hope this will suffice your query.
This entire thing is what we call a storm in a tea cup.
It's a waste of time (yours, in thinking about it and being stressed)
1. No employer can force an employee to work when he does not want to. The law forbids it. At most, you need to serve notice period. If no appointment letter is given or nothing specified, you need to serve 1 month notice pay.
2. As long as you have resigned and have served notice period, you are free to join any company irrespective of whether the resignation has been accepted.
3. Your employer can not impose any restriction on your future employment, whether you join a client or a competitor. No agreement or bond signed to that effect has any validity.
4. As long as you have an appointment letter from the new company clearly stating they are appointing you, you need not worry.
5. Do you think the CA has nothing better to do than sue you in a court of law ? They have better things to do in life. Don't worry.
However, it's always better to have a record of the fact that you have resigned.
Best you follow the procedure nipuna suggested