An address can only be attached to one Mailman user, so if for some reason an address is registered multiple times, only one Mailman user can have it.
A CO Person can only have one active address, since the preferred address is used for Mailman message delivery.
Mailman Users span domains, so in a multi-tenant instance (or a single tenant instance where more than one provisioner is configured to provision more than one domain), each tenant will likely require its own mailman server, not just its own domain.
Registry assumes it has full management of Mailman Users. If an email address is directly added to Mailman, and that email address is subsequently added to a Registry record that is then provisioned to Mailman, the provisioning action will fail, since Mailman will indicate that the address already exists (but Registry won't have any knowledge of it).
Renaming a list is not supported. Changing the list name in COmanage Registry will not change the list name in Mailman.