Since you seem to be offline, ill leave a response here which we can discuss later on
Well from what you are describing, one thing seems clear while she may have been depressed certainly, depression by itself does NOT lead to dementia. It is likely that the dementia is due to a seprate etiology which was either missed on the earlier evaluations by the neurologist or more likely developed after the evaluation. The speed with which her condition has deteriorated means that this was a fast moving disease which impaired her mental capacity. The first one which comes to mind is Pick's disease which is also known as fronto-temporal dementia, which can cause hallucinations among all the other symptoms which you have mentioned. Secondly, it is also possible that she may have developed a variant of creudzfelt-jakob disease which is essentially an infective disorder caused by an organism which is known as a prion. This can rapidly cause a person to develop dementia and can also cause muscular jerks which are known as myoclonus. Given the situation and the severity of her symptoms, it is very unlikely that the dementia could have been prevented no matter what you would have done. Depression can occur early on inn CjD and the problem with this disease is that it is irreversible. Same is the case with Pick's disease, both of these disorders even when diagnosed early cannot be treated effectively and the patient invariably deteriorates over time to develop dementia.
Now it is possible that she could have developed dementia due to other causes including vascular dementia, lewy body dementia among several other issues however depression by itself is NOT known to cause dementia. It is possible however that she had a disease which caused depression and then that disease led to the dementia now such as CjD which i have already mentioned. Depression is actually a symptom of an underlying disorder and not a disease by itself (except for major depressive disorder which is purely a psychiatric issue and NOT known to cause dementia).
So to answer your question, no this could not have been prevented or delayed if she has any of the above conditions because usually these disorders are irreversible, even if you got her a job, therapy or anything else, it is likely the outcome would have been the same. Im sorry to hear about your mother and i hope this answers atleast some of your queries.
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