1861 census has Catherine Morrison age 90, formerly ag lab, in South Boidsale, SouthUist, living with her daughter and son-in-law, John and Mary McLellan and born South Uist.
1851 census has Catherine McLean age 80 born South Uist as a pauper formerly seamstress, a visitor with the MacIsaac family in South Uist.
In Scotland it was not uncommon for women to revert to using their maiden name when widowed. So it looks as though Catherine was born Morrison and then married a McLean. As for her death certificate - her nephew couldn't read or write so couldn't check whether the death cert was accurate. If the registrar has asked "was she married?", nephew may have said no meaning she was a widow, but the registrar took it to mean never married.
It's a possibility. But you'd need to try and find a marriage record, plus baptisms of her children.