Hello Wishing,
I can relate to your post. My Grandma had Alzheimers, Parkinsons, and a few small strokes. I took care of her from moderate-severe. It was very difficult for me emotionally. When grandma came out to me, she could no longer talk. Her face was expressive. But she couldn't understand everything people talked about. Grunts, head nods, and expressions were her only ways to communicate.
That was probably about 13 years or so ago. I don't think that I will ever forget sweet incidents that occasionally occurred. Those are the sweet memories that need to be held onto. Years later, I became a CNA and worked at a nursing home. Caring for others needing assistance is a passion that I've held onto.
I am now disabled myself, but I still have things that help me fulfill my desire. Volunteering as a friendly visitor to the elderly and respite care helps me and it helps them. I can relate to their problems in many ways, someone being there to talk with or play games and look at pictures together can be a real lift for them!
I'd recommend looking into elderly county programs that exist where your parents live. I work for Elderly Network, which does have different programs & resources which are available. They don't just accept regular Joe's off the street to help. They do look into volunteer background (for safety), and try to match personality styles to make good matches.
Oh ~ one other thing. YGrec23's comment about going over HIPAA with your mom now would be especially helpful! So many times, doctors and nurses can't share any information with family members because HIPAA laws (there to protect personal info). Your mom can still decide who she trusts to be involved in her care. Your dad can too. Then, educated decisions can be made as the disease progresses. And you won't have to wonder or feel guilty for decisions made.
Gentle hugs and best wishes sent your way ~ It is such a rough road at times.

You're in my thoughts