I am so sorry for the pain you are going through. I certainly can understand; my mom has Alzheimer's, but for her, it has been a very slow process, now going on five years. I find it extremely painful.
I'm sorry I can't answer your question about a stroke causing Alzheimer's but I can tell you that Alz can be a very fast process, going through the stages very quickly.
No matter, it is a loss and when someone has Alzheimer's the family experiences a loss while that person is still alive, because they simply are no longer the same person. My mom really has not been "my mom" for about four years. Most of the time, she does know who I am. Like you, I spend my time with her trying to "bring her back"....it is so hard to accept their world, even though I have read it is better to step into their world than try to bring them back into ours, I still find myself desperately trying to bring her back.