Does a diagnosis of Dementia always lead to Alzheimer's disease?

There are many causes for dementia and ALZ is different from dementia. Albeit the progression of both diseases is similar in that they both are responsible for cognizant impairment. Strokes can accelerate dementia but not always and some have strokes that make them mentally impaired but that doesn't mean they have dementia.

Your doctor or neurologist specializing in geriatric disease can do a full work up, tests, scans to sort out if you want. They can sometimes distinguish ALZ from dementia. You might want to check The FDA and NIH website as they have considerable info and research on dementia and ALZ that may be of help to you including clinical trials.

I've had three friends whose parents died from Parkinson's yet did not have ALZ or dementia. As the disease progressed, their speech was more difficult, one had mor difficulty focussing on tv, newspaper, books, or long conversations, one had less interest in conversation or books, yet still enjoyed short reads and newspaper articles. They had a little more difficulty following a conversation or storyline when they were tired.

Hope this helps. I don't think even drs always have a clear understanding of how dementia differs from ALZ and there is still a lot of ongoing research.

My mother's had Parkinsons for 15 years and dementia perhaps the last 6 or 7. She's also had a number of strokes over the years. I've been reading but I can't figure out if the dementia is caused by the other issues or something in and of itself.

No, you could die of pneumonia, get hit by a bus or be shot by a jealous young wife. Take my MIL, she has dementia, kidney failure, congestive heart failure and a history of strokes. At 86, anything can happen and often does. My MIL wants to die peacefully in her sleep. I told her " You will die peacefully in your sleep, but everyone else in the car will be screaming". ( We took her car away shortly after that). No man knows the place and time, only God.

But it always ends the same..., right?

No, there are many different kinds of dementia, but a good Neurological Clinic can sort it out.