What happens to security workers with dementia?

The book does not say that the agency does that; characters speculate that it might.

Isn’t it easier to just have the pilot wear a gorilla mask and a bowler hat?

I’ve heard of military pilots encountering classified aircraft (and subsequently being sworn to secrecy), but trying to get an entire passenger jet full of civilians to not tell anyone what they saw seems like it a logistical nightmare, and also totally unnecessary. Aren’t there areas of restricted airspace where test planes can be flown without being seen closeup by civilians?