As to dementia, symptoms can be subtle and people can appear fairly normal while still in their same environment.
When I worked in inpatient psychiatric hospitals in the San Francisco Bay area, we had some solo travelers admitted with dementia-like symptoms (and other solo travelers who were manic). Sometimes just getting outside of their regular routine would worsen symptoms dramatically. If there's a brain insult (such as a new drug, even antibiotics, or, say heat) the symptoms can come on pretty abruptly and may not completely subside after the drug is withdrawn or the brain is cooled down. It's possible Barbara had (fairly) early onset Alzheimer's. It has crossed my mind. One doctor told me that it can appear sudden to family members, but also that in the elderly, symptoms can appear to onset 'overnight' due to new meds, emotional upset, fever, illness, etc.
My dad had this happen (although he was very old) due to prescribed antibiotics (literally overnight, he went from typically very lucid to completely confused about where he was, who people were, etc).
To me, this theory is stronger than the abduction theory (but of course, there are other theories as well).