Is it elder abuse to purchase a home at way less than market value from a parent with dementia?

Sara, it depends on how far advance the dementia.   If the elder is still able to understand the steps involved in selling the house, and is selling to lets say a grand child who is just starting out with a young family, then selling low is not uncommon.

It also depends on the elders financial portfolio, if the elder has enough money to tied him/her over for many years, and moving into Assisted living or a nursing home is no problem budget wise.

Now if the elder is far into their dementia journey and has only a few thousands saved up for their future care, then there would be an issue if Medicaid is needed with a 5 year time span.

It certainly won't fool anyone like Medicaid or tax authorities. Bad idea, if that's the plan.

Yes. Reportable to Adult Protective Services as financial abuse.

Yes it can be,and it can disqualify the parent from Medicaid benefits with a transfer penalty equal to the amount below market.