Somebody recently told me they’re not worried about dying, they’re worried about the run up to it. It’s that run that we’re talking when we say “frail”, “fragile”, “Sunshiners”. The huge challenge is that what’s going to happen is pretty well completely unpredictable. So you have to be ready for ANYTHING, almost.
Lately, my wife’s parents have become Sunshiners, both experiencing memory loss and personality changes of dementia. Because my parents both died relatively young and didn’t experience frailty, helping my in-laws is for me a new experience. And believe me it’s very much different to live it with family members than it is (as I have for many many years) deal with it professionally. It reminds me of how much more real delivering babies became for me after we had our first one!
It seems to me there are two parts to staying out of trouble. The first one I’ll call “surveillance”. Sunshiners live from day to day like everybody else, but they are unlikely to remember and tell you about a lot of what’s going on. So somebody has to check on them regularly. How regularly depends on how frail they are, and you have to know how long somebody can be left alone. For my in-laws, it’s about two days right now. For some behaviorally troubled very demented people, it might be almost no time at all.
The second part of avoiding disaster I’ll call “intervention”. This is when you do what you have to do to pick up the pieces when something goes wrong or changes. But what is that something likely to be? The common things are a minor illness (flu, urinary tract infection, etc.), a minor injury, reaction to a drug, or a behavior outburst. There are also major illnesses like heart attacks and strokes, and major injuries like hip fractures. But what you are looking at might not be that easy to identify. Dad or Mum is just very strange or very different all of a sudden. In that situation you need someone to help you find out what’s going wrong and what you can do about it, and you need that help RIGHT NOW, not tomorrow.
That help takes the form of good solid trusted friends and family members, and also available healthcare providers, preferably working in a team. All of which may be hard to find. But it’s worth the effort.
Sunshiners: Frail Old People Living At Home 
My first thought on reading this was: I am so sorry. For your in-laws, for your wife, and for you. You are all heading in the direction of nightmare country.
The short answer is that you won’t prevent disasters, they will come unless the in-laws slip quietly away at home before they strike. And one day hindsight will be cruel to you. I am sure of this although none of us yet know the details.
As you say, the team of carers will include professionals, plus close family and friends who have different and vital roles to play. If the dementia deepens, and the elders are fortunate, their family and friends will continually supply memories and context to the sunshiners’ lives. This is a very, very close relationship; you may be spending a lot of time as if you were inside the sunshiner’s head, and yet without any control. It is very taxing.
Go on this family journey together with your loved ones, it will be one of the most profound human experiences you ever have.