From the dental aspect, there seems to be several contributing factors. Looking in depth, mercury can enter the brain, destroy cells and move on to the kidney or other organs for refuge. When one looks at the brain, yes, there is more mercury present in the brains of people who have amalgam fillings. That does not assure one of coming down with Alzheimer's however. Haley's work has shown that small amounts of mercury added to the drinking water of experimental animals (rats, I think, but I'm not sure), can produce some of the characteristic changes in brain tissue. It certainly looks like mercury can be a contributing factor, but it is probably not the only culprit involved.
One of the notable changes in an Alzheimer's brain is the loss of beta tubulin. Tubulin is part of the "skeleton" of brain cells so to speak and this disappears or is greatly reduced in the Alzheimer's brain. Tubulin is part of the transport system within each cell, so interference here is of high significance. Chemicals extracted from root canal teeth have been added to brain homogenates and the tubulin has been noted to be reduced in some people and to totally disappear in others. Since reduction of tubulin is a characteristic of Alzheimer's it might be worthwhile to consider the presence of root canals in people with Alzheimer's.