Are senile dementia and alzheimer's nearly the same disease?

Controversial. For some drs. They are the same. Some neurologists make a difference based on the age of presentation ( the older most likely "senile" ) and probably other features. The term alzheimer's is reserved for most insidious earlier onset. For a detailed discussion... .
Yes. "dementia" is a general term for a decline in cognitive capacity from what was previously achieved (unlike developmental disabilities.) "senile" means it occurs after the age of 65. "pre-senile" means before the age of 65. Alzheimer's, or "senile dementia, alzheimer type" is a particular pattern of dementia, distinguished from vascular dementia, fronto-temporal dementia, or other dementias.
No. Alzehimer's is also called "pre-senile" dementia to show that it is not related just to aging. There are differences in the pathways of how the diseases develop. Plaques are found in the brain matter of alzheimer patients that are not seen in senile dementia. You could develop both -- but they are not necessarily equivalent.