Mail Merge from Numbers works just as you are trying to use it.
You create a Numbers document with the proper layout for merging.
Enter the data in teh numbers document and Save it.
You create a Pages Document to merge into and create the links to the Numbers document.
You Edit > Mail Merge in Pages.
To merge again from the Numbers document, make the necessary edits, Save the Numbers document and do the mail merge again in the Pages document that you created for that purpose. The only problem that normally arises is that we forget to Save the Numbers document after editing.