FIM/MIM Galsync between two Exchange Orgs where contacts already exist

Case Overview

Contoso has two exchange organizations, Contoso and Fabrikam. Contoso currently manually creates contacts for Fabrikam mail users and vice versa in Fabrikam. They would like to improve this synchronization process by using MIM Galsync. There key technical requirements are

1. Currently the existing Fabrikam contacts are scattered all over the directory in different OUs, Contoso would like MIM to discover these existing contacts and place them in a designated OU. There are also duplicate Fabrikam contacts and invalid contacts, by MIM discovering the actual valid contacts, Contoso can identify and cleanup invalid contacts. The same request also comes from Fabrikam.
2. The existing contacts should be joined to the mail user accounts so that there is no disruption to the business, these contacts have already been used in the directory.
3. Mail user accounts that are not already synchronized should be created by MIM as contacts in the other directory.

Proposed solution