Exchange with active directory and child domains

If you use a DOMAIN LOCAL group, you'll be able to add the users from both domains to the one group.

Better still, add a Global/Universal group from the BRANCH domain (and the same from the CENTRAL domain) to the Domain Local group:

CENTRAL DOMAIN
SPECIAL GROUP
CENTRAL\Alice
CENTRAL\Bob

BRANCH DOMAIN
SPECIAL GROUP
BRANCH\Carol
BRANCH\dave

CENTRAL DOMAIN
JOINT DOMAIN LOCAL GROUP
CENTRAL\Special Group
BRANCH\Special Group

That way, adding new members to the JOINT group just means adding them to the global group within their domain.

Because you are seeing replication errors there is the possibility that doing a dcpromo would not cleanly remove the child domains.

Doing the metadata cleanup as described in the KB article will remove all traces of the child domain from your parent domain. I would make sure the domain controllers for the child domains are powered down or rebuilt without the active directory role before doing the metadata cleanup.

This all assumes there is nothing in the child domains that needs to be retained.