re: Anyone know the reliability of the on-line memory tests for dementia risk?

They are just very basic screening tests, and taking them online is practically worthless. Of course, a neurologist won't give much beyond that testing wise. Ask the neurologist for a referral to a Neuropsychologist for comprehensive testing. Best case scenario, she performs in the normal range and now you have a baseline by which to measure any potential changes in the future.

52 is really young for Alzheimer's, but there are some types of dementia which can have an earlier onset. Good luck.