The research seems to show that mild hypothermia slows down neural activity in the brain but does not damage it. Mild hypothermia is sometimes medically induced after stroke or TBI or protect the brain. 

For those people with Alzheimers or dementia and hypothermia, I think their brain has other problems, and hypothermia is downstream of those other problems.

I explicitly remembered hearing it, it was one of his recent book tour videos, but I have looked back and have not yet been able to find the timestamp

You are right that the scalp is less than ideal. The forehead, neck, insides of the nose, and I think the upper parts of the cheek are all better for cooling the brain. I believe the best method found is one where cold dry air is pumped in one nostril and out the other to greatly increase evaporative cooling from the inside of the nose, which cools the blood moving into the brain

A water cooled forehead combined with a water cooled neck pad might work well

Re: Newton's law of cooling - I'm not sure if it applies as much to the brain.

This is because the brain has other cooling mechanisms that kick in when overheating is detected. Yawning, panting, sweating/evaporative cooling from the forehead, and also blood flow to the brain can increase as well. 

Yeah - you are right the cooling curve is definitely not linear - but I stand by my point that the data clearly shows that the brains heat generation ability greatly outstrips its cooling ability.