As someone who litigates for a living, all of this seems overblown to me.
Much younger witnesses, often pretty accomplished ones, misstate dates and names multiple times during the course of a lengthy deposition. People do it in everyday conversation all the time. Sometimes the questioner and the person being deposed briefly become ships passing in the night because of a misunderstood question or context. These nits usually get cleaned up on the record, but sometimes not. It doesn't mean the witness has dementia or is lying.
Likewise, "I don't recall" answers don't mean the witness has dementia or even a bad memory. Unless a witness is a corporate representative who is required to educate himself or herself to speak on behalf of the company on previously designated topics, witnesses are not required to "study" for a deposition. Their lawyers typically instruct them to tell the truth but not speculate on matters they do not recall. You would be suprised how much we all cannot recall about things that happened three or four years ago. I have recently been working on a large commercial case with lots of smart, accomplished witnesses aged in their 40s to 60s. We sensed that the other side was giving too many "I don't know" and "I don't recall" answers on basic topics. We counted them up, and they came close to a thousand. But when we counted such answers by our own witnesses, it totaled about half that--still a substantial number. None of these witneses have dementia.