How does someone from County Adult Protection check on unable seniors living alone when they won't answer door for strangers?

Pop52105, so sorry to read that you are still having issues with your brother and his wife. I remember you coming on board here back in early June.

Wasn't your brother in Rehab for awhile? Curious how he convinced Rehab that he has care at his home. Of course I remember my own parents [90's] saying they take care of each other.... [sigh].

So if I understand your question, you are expecting APS to call on your seniors and you don’t think they (APS) will be able to gain access?
I suspect they are accustomed to having this challenge. Often seniors will respond to folks who appear to be in authority when they won’t cooperate with others.
If the APS has their phone number, they might call first and explain who they are.
You could ask APS what steps they are willing or empowered to take. Perhaps they call the police or fire department to gain access.

You can all local police non emergency and ask them to go to check on her welfare. You may want to persuade her to purchase a device that alerts EMS.

Is it possible that you could give a neighbor the key, or give one to the local police dept? Sort that's al I can think of, except maybe install a monitor inside hr house to do video surveillance? It sounds awful, but will let you know if emergency services needs to be notified for her.

I have read that if you are worried about someone you can call your local non emergency police number and ask for a wellness check.