Regarding the issue of Zentyal saying that the user already exists, when I originally installed Zentyal, during installation you had to provide a user name.
After installing, this particular username, (although it existed as linux user and as a user that could login to the Zentyal administrative interface) it was not added as a user under the Office > Users and Groups > Users.
When I tried to add it there too, it would say that the user already existed, even though it was not listed under Office > Users and Groups > Users.
So, were you trying to add a user with the exact same username that you created during install? If so, you may want to try another user name and make the user a domain Admin.
I'm not sure this will solve your overall goal, but it may address the "user already exists" portion of your issue.