I can only recommend having an attorney you can call who will threaten those who won’t respect the legal authority of the documents. People are always the weakest link unfortunately.

Lawyer. Passport. Locksmith. Gun. (A Talk About Risk and Preparedness) [video] - https://news.ycombinator.com/item?id=33509164

(I hold POA and medical proxies for people who need someone they can trust to assert their medical decisions and wishes for them when they are unable to)