First of all the government department, if there was no rival claim i.e., if your father no other wife other than your mother surviving his death, cannot ask for succession certificate especially for grant of pension. It will be family pension and she being a legally wedded wife should get it automatically.
I think you have not given complete details of your case.
Instead of succession certificate your mother can get legal heir ship certificate from Tahsildar's office or revenue department.
Since your advocate is not handling the case as per proper procedures, you may even change the advocate.
Actually for obtaining family pension, proof of legally wedded wife shall be sufficient and production of succession certificate has not been mandated in law.
There is something wrong in this hence you may clarify on this.