Conflicting permission sets when working with shared or delegated folders in Outlook

Are you only talking about Exchange On-Premise?  In Exchange Online:  You can set delegate permissions and view private items thru powershell using the set-mailboxfolderpermission or add-mailboxfolderpermisson commandlet on a users calendar folder with the -SharingPermissionFlags parameter.    

https://docs.microsoft.com/en-us/powershell/module/exchange/mailboxes/set-mailboxfolderpermission?view=exchange-ps

There are also options of limiting what a user can do in terms of permissions using GPO.   In addition, only an administrator can give full permissions to a mailbox..a user cannot do that.  In a mailbox with lots of folders it does not make sense for a user to give access to each one separately.