How exactly could one possibly prove someone remembered something at a specific point in time short of their stating so at a later date?
Well, she earlier claimed that she had something to testify to that could potentially incriminate herself, so she'll have to be able to remember that.
That's a brilliant first question - tell us what you did that was illegal!
"As for asking her what possible testimony she thought might incriminate herself, she still has the attorney-client privilege."
This I doubt. She's waived that privilege by invoking the Fifth Amendment. Once waived, she can't reassert it.