Setting UntrustedAppsPolicy to DISALLOW_INSTALL does not prevent app installs

For anyone facing this in the future....

As it turns out,  that allowing a user on Android Enterprise to add a personal account seems to override the restriction preventing the user from installing apps that aren't white-listed. I am not sure if this is an Android Enterprise defect or design, but at least I've been able to prove it by testing on 2 different devices under the different conditions. 

Thanks much to everyone that reached out to help.