You are right Hill.... every one is different. My mom is angry at the world and spits venom frequently along with being stubborn and hard to deal with. My Dad is usually a sweet heart. They lose their judgement, ability to think logically, and the social graces they have learned early. That leaves behind some that are mild mannered and some that are just plain difficult. It is not something they do intentionally, just something that happens.
As for being between stages... absolutely. It all depends on what part of the brain is effected and when. Mom has manic behavioral changes, her judgement is shot, and her ability think logically is fading fast. She has totally lost her abstract thinking (number in particular) and is losing her ability to retrieve words. She dresses inappropriately but........ she still gets up every morning and takes her shower. She refuses help and fights what she doesn't understand. Dad, he's definitely an oddity. He doesn't know where he is or what year it is. He can't remember anything short term. He wants to go home to help his Mom milk the cow and has hallucinations. Yet he remembers my face and calls me by name. He refuses to shower but he will fix that hair to perfection. He has times of totally inappropriate behavior and incontinence, yet other times you wouldn't even know there was a problem. Dad loves to have help and accept whatever comes his way.
So it's not suprising that Mom and Father in law are different. Each is displaying behavior related to what part of the brain is affected the most and rooted in their basic personality. My best guess is that your Mom has always been a sweety and your father in law has always been a bit of a pistol!!
You are doing what you can. All you can do is be there when the family calls and do your best while you are there. There is no way to logically have a discussion with FIL to make him different. It is what it is and the best you can do is find little tricks and techniques for dealing with his behavior.
It is known, that for some, Namend causes behavioral changes that are not pleasant. If the doctor agrees you can cut back on his dosage or eleminate it totally for a short time to see if the behavior changes. If not he can go back on it. If there is a dramatic improvement... then you know
But that decision will have to be left up to the family and doctor. There are others on here that have tried it and I am sure they will tell you of their experiences.
Hang in there....