Hi @suerc, you'll notice that I moved your message to the Caregivers group because I think you'll get more responses here. You ask some very good questions and I'd like to bring in some other members like @debbraw@rmftucker@maryterry and @julesa who may have some thoughts and experiences to share about people with Alzheimer's being on their own for periods of time and when or how to manage responsibilities they may wish to take on.
If I'm reading your message correctly, I see 2 separate but related issues. 1) You're concerned that your neighbor who has Alzheimer's is being left on his own because his wife is also juggling long distance care for her father, and 2) you wonder if and when your neighbor may not be able to manage caring for your pets when you're away – a responsibility that up to now he always did.
Do you think that his wife is aware of his declining mental abilities? It can be a delicate discussion to brooch as she may be in denial and not wish to see it, especially since it will force her to have to make choices about her father's and her husband's care. Not easy.
It is possible that the responsibility of caring for your pets may lead to a source of anxiety for your neighbor. I wonder if you might bring up the topic with his wife and share with her your observations. This may be an opening for her to recognize that she too has to put some things in place for his care in her absence. Sue, do you think this is a topic you can discuss with your neighbor's wife?