Filing return on behalf of deceased

My mother died in February 2018. I am her sole Legal Heir. Because of my posting in Darjeeling i have been unable to obtain the Legal Heir certificate till now. Income tax authorities don't accept a sworn affidavit anymore. My mother was a Super Senior Citizen with total income below 5 lacs. My queries are: is filing the tax return on her behalf in view of her super senior citizen status and income below the threshold limit mandatory and will the income tax authorities impose a fine of 5k if i file the return after July 31 ? Thank you