Well, I'd be a bit nervous choosing the questions to ask an 84 yr old, for proof of life.
Believe me, even if she is still 'sharp as a tack', her memory aint what it used to be.
So remembering what her doggie from 75 years ago was named, might backfire if she can't remember anymore. Or even what she ate at her daughter's last week. It's been a very stressful week.
I'd go with things that she'd have a better chance of remembering---"when was Daddy's birthday? When is your anniversary, etc? I think she could have a greater chance of remembering something significant like that. And I doubt her kidnappers would know those dates.