GWT,
First let me offer you my heartfelt sympathy on your circumstances. Alzheimer's runs in my family, on my dad's side also. So when you have someone in the generation above you suffering, it hangs in the background of your life. The more often you have a "senior moment" the more often you question "is this normal or a sign I am in the beginning stages of the disease?"
Insurance companies, by and large, know they have the upper hand when things like this happen to their customers. Best advice I could give is get a lawyer to advise you. It would be money well spent.
No I am not a lawyer, and no I am not pushing for litigation here. But if you can get some expert advice, you might find your stress level goes down on this issue. Plus, having someone help you with this can keep you from making disastrous mistakes.
Being fifty in my book does not make you anywhere near an old goat. It's never the age, it's the mileage that matters.
I have had situations in my life where I was dealing with stress on overdrive, too. My dick sometimes did not respond the way I wanted it to either. Sex is 99% mental. My mind could not focus on the fun at hand, so my dick did not respond either. The fact you are off your game sexually is a very strong indicator you are overstressed at this time.
But sex and just being intimate with your partner can be the best antidote to stress there is, just as long as you allow yourself the option of not having to perform at previous levels.
Viagra is not going to be the answer because it requires you to be mentally in the mood. You don't have ED because you can get erect, but it just doesn't have the hardness it did. And the onset coincides exactly with the onset of major stress.
Find a way to get away from all this for the moment. Even moderate exercise will do wonders. Take a long walk in a park with your partner. A change in scenery would do you both good. A massage does wonders for relaxation. And get a good night's sleep. If you are not sleeping well, it can make everything, including your sex drive, worse.
Another thing, realize you are probably dealing with some mild depression also. You have to mourn the loss of your home and the years you have devoted to it. Alzheimer's forces you to mourn the loss of the person long before they are physically gone. And putting your dad in a fulltime care facility can trigger both mourning and depression.
One other thing, your partner is dealing with all this stress too. Don't forget to take care of him. Now is when you really need each to be there for the other.