have a question from user perspective. If using a shared mailbox and give full access for multiple users. Could one user see the permission of other users? Sure in the Mailactivity, but is there in the Browser or Outlook Client a option to see this?
I'm 99.999% sure there is no way to see that from the user perspective. The one exception I can think of is that users can for themselves look in Outlook's Test AutoConfiguration tool, under the XML tab, at the bottom of the output, see any <AlternativeMailbox>...</AtlernativeMailbox> for mailboxes to which they have FullAccess and where AutoMapping is enabled. But that is still limited to something you can see for yourself, but not on behalf of others.