A few questions to better understand what you are asking.
In answer to your first question, I am actually not sure, but someone from Kentico can probably answer that
In answer to your second, When you say you want to block admin access for the newsletter, are you saying you want to have an admin that can manage subscribers, but not use the administrative interface? Or are you saying for your users to view the login protected they are also getting edit page permissions as well?
If its the first you could do one of a couple of things. If you have UI Personalization you could set it so the user only has access to the Newsletter Module under UI Personalization > CMS > Administration > On-line Marketing > Newsletters OR you could build a custom form to give the user access to the manage subscribers.
If its the second need, then its a permission issue. You can have content secured behind a login where the user doesn't have admin privileges. I would check the permissions you are giving the user and the permissions on the page. You can read more here: https://docs.kentico.com/display/K8/Configuring+permissions
Not sure if this helped, but some more clarification might allow us to help out more