I agree with KevinI - my mum's scan they said normal age related shrinkage, and didn't show any thing else. They didn't do any further scans, from reading TP these only seem to be done for those who are younger when symptoms start, my mother was 84 at diagnosis. She also did very well on the tests, scoring at a level at which indicated no issue whatsoever 27 out of 30.
However by this stage with hindsight she had clearly not been right for 3 years, probably needed intervention to be kept safe for 18 months before the crisis, with odd little incidents going back 5 to 7 years.
The diagnosis of Alz was made purely on the observations by brother provided to the nurse and consultant, and how she presented to the nurse who visited her on 2 separate occasions at my brother's house. Things like being unable to explain why she had turned her electricity off, and saying it was because they were stealing it, but then changing her story, were fairly clear indications that things weren't right to GP and memory clinic nurse.
As canary recommends take a letter detailing everything and hand it over separately.
From GP visit to diagnosis took 3 to 4 months, much of this with hindsight was spacing out times of nurse seeing her to see if her symptoms were stable or not, which I think is to eliminate other things and also to determine the type of dementia.
To some extent the brain scan is more to rule out other issues than confirm dementia.
If it is Alzheimers then medication is available, if it is deemed another type then no specific medication is available, although many PWD benefit from anxiety and depression medication as these symptoms are quite common.
Sadly it is then a case of managing symptoms and behaviours, both for the benefit of the PWD and any immediate carers, such as your dad, who is clearly having to support your mum, and deal with her obsessions.
My mum was diagnosed formally 5 years ago, and is still classified as in early stages, although there has been a slow deterioration in the 5 years.