[mmorse]

Welcome to the forums,

You could have just left that mail.domain.com there...

su zimbra

This will create an additional admin account with the password of 'test123':
zmprov ca testadmin@domain.com test123 zimbraIsAdminAccount TRUE

This will transform a normal existed user account into an admin account (in this example futur-admin will become an admin) :
zmprov ma futur-admin@domain.com zimbraIsAdminAccount TRUE

To change the password run:
zmprov sp testadmin@domain.com NEW_PASSWORD

once you try 'zmprov gaaa' can show you if it did become an admin account

EDIT:

I also ran zmprov ma admin@domain.com zimbraIsAdminAccount TRUE which gives an error: account.INVALID_ATTR_VALUE (zimbraIsAdminAccount must be TRUE or FALSE)

sounds like the sticking point
what if you delete that user, then create it as admin off the bat ('ca' instead of the 'ma' method) ?

So did you migrate a bunch of users to this or could you just reinstall?