Give them this document with pleasure as this will have no legal sanctity or liability for you.
Ask kindly to be provided with relieving letter and Experience letter + Gratuity.
If company is unwilling, tell them that you will have to approach labour court.
a) legality of company asking me to sign such a bond now after leaving the job
This is not legal document ,This is so vague that the companyneeds to attach the list of customers,Competetiors or business partner.If they produce and you sign on all the lists then they will send you a legal notice .
b) validity of clauses in NDA
Since this is not a legal document this would be in valid.
c) ways to secure relieving letter and payment of dues.
Simple way is to sign the letter and reply them by post for a list of customers,Business partners and potentil business partner.