Mom (86) accuses dad (91) of affairs, taking her things and she thinks she gets phone calls saying "please die". What to do?

Who is the POA? Do you BELIEVE that neither parent has any diagnosis, because if not you re dealing with mental illness. You don't say how long this is going on. Your father, given he has no diagnosis and you find him to be present, alert and rational is in charge of what further is to be done here. You are nearby. Stay alert. You can speak to doctors, however, they cannot speak to you appreciably without a POA for health care, which is something you should all have in place now for both parents. Often, if you speak with doctors about this they will give you clues. Such as "You know I cannot recall all this testing you are speaking of, but couldn't speak with you about if I HAD done it; are you POA for your parents? Would they be able to assign you such status. You can certainly let me know your concerns but do know I cannot discuss their medical history with you without their permission."

You listed your post under Alzheimer's/dementia, so I'm guessing you don't believe that your moms tests came back normal. The paranoia she is experiencing is very typical of those suffering from mental decline. There are medications to help with that, so probably best for you to take her to a neurologist so she can get the help she needs, and your father can live out his days in peace. Best wishes.

Why do you believe her test results unless you’ve seen them for yourself? Is there a history of mental illness or is this new behavior? She needs a neurological evaluation that you witness and hopefully meds to calm the behaviors