Sorry, I can't really help further
I already tried using different wording and sentences to explain what domain and A or CNAME records are and how URLs use it, including the default domain idea but I don't have any other idea that could make you understand if this is not already clear enough.
Perhaps someone else with better English and different wording can succeed
On top of this, although you don't clearly state it, you are making things even more complex because you do not focus on internal behaviour first (I mean access from you LAN) but try to check it in parallel from internet that will require to modify external DNS content (assuming, which I strongly hope, that your Zentyal DNS is not exposed and used on internet).
My very last suggestion is that you stop any change for the time being, do your home work to understand a bit more about DNS then read this whole thread again with this new understanding before coming back on Zentyal interface.
Sorry