First of all, i personally don’t have practical experience with using Openfire on internet or on a main local domain. So maybe someone else can comment on that. I know @Guus is planning or is already hosting Openfire, so maybe he had to deal with that. @speedy is our AD\Windows network guru, so maybe he can give advice regarding the local AD network.
In my understanding XMPP was born as a global network to live in internet and compete with AOL/Skype/etc, with federation in mind. So its design is more suitable for such global deployment. Although it can be installed as a local chat, it still has the same requirements and constraints.
As about pointing to the main domain. Same is with email. You can have AD domain abc.com, a website www.abc.com and emails with firstname.lastname@example.org addresses. Your Exchange server might be mail.abc.com, but email addresses doesn’t have to be email@example.com. And you can manage this with DNS (SRV, MX) records. Say this site has a website on igniterealtime.org and the chat server on the same name (i put igniterealtime.org in Spark to login). Although i think its server’s FQDN is probably xmpp.igniterealtime.org. It could be that its using the same cert for both website and chat server. But maybe Guus can comment on that.
For client connections you can manage this with SRV records like:
_xmpp-client._tcp.abc.com. 18000 IN SRV 0 5 5222 chat.abc.com.
Not sure what to do about Admin Console though. This is not a part of a global XMPP design/standards. Servers’ developers had to come up with something to administer servers. Openfire uses web console. Some use cmd or config files.