I am specifically asking what happens to the items in an Inactive Mailbox that was placed on Litigation Hold prior to Deletion when the Litigation Hold has say a '6 mo.' Retention Period assigned.

Do any of those items 'expire' out of the mailbox?  Do only the items in the 'Admin' folders of the Deleted Items get removed, or none?

I am not asking about recoverability of that mailbox, more of what the behavior is specifically when the Litigation Hold is set to expire after a certain amount of time.

There is no expiration, check the blog post here: http://blogs.technet.com/b/exchange/archive/2013/03/21/preserve-mailbox-data-for-ediscovery-using-inactive-mailboxes-in-exchange-online.aspx

All content in an inactive mailbox is on hold until you remove the hold from the mailbox.