1. You have rightly been advised to apply for a fresh succession certificate. The succession certificate issued in 1990 cannot be amended now.
2. Your mother died in 1977, whereas the bonus shares were issued in 1982 and 1986 respectively by the company. You could have sought these bonus shares when you applied for succession certificate. Be that as it may, the court cannot now amend the succession certificate. You have no option but to apply for a fresh succession certificate.
3. That two other legal are deceased whereas the remaining cannot be coordinated with is not a ground which emasculates your right to apply for a succession certificate. You can as one of the legal heirs of your mother seek succession certificate.