Age related mental decline affects everyone. This lady is at the, “it couldn’t possibly be my fault, I still have all my marbles” phase. Returning items is super common because there’s no way she could have made a mistake, it must be your mistake. Stand your ground, those tears are not your fault.
More like probable. These things go on for years before a single event or crisis forces action by the family. The stereotypical getting lost while driving to a place they go to daily is usually it. I had a great aunt that put the entire ice cream container on the stove to cook it and started a fire. This was at Christmas dinner. I’m sure her kids had been patching holes in that shop for months but on that day it finally sank for all to see. She passed about a year later in full blown dementia.