I think you are getting a bit defensive in your comments. I really appreciate your help but I think I need to make a few things clear.
J5 is beta, very beta, so the docs aren't the best in the world, however if you are familair with any of my apps you would know that nothing works out of the box. Besides a little common sense is required... the viewer reads ctrlr files. How is it supposed to know where your ctrlr files are or which ones you use without pointing to them? There is a readme btw... it's called readme.txt and its' in the root folder.
I expect users to play around with things just a tad before they start asking questions.

No the viewer doesn't assemble an image. The image is pre-rendered with labels overlayed on top of it. Again, I can't believe you went to all of this trouble without experiencing the viewer yet. By the sound of what you are wanting as opposed to what the viewer does you might find out that you don't like it that well. I'll stress again how much I appreciate it, it just worries me a little that you made something work for an app without ever really using the app.
As I said... default options never work. As a matter of fact default options never work on nearly every complex application in the history of computing.

I'm sorry you feel that having to change a single control will keep people form using the viewer, but nothing could be further from the truth. Judging by feedback and comments I get pretaining to this application it's seconded in popularity only by dragon king itself. And keep in mind, it's beta and has never had an official public release.
This type of applicaiton simply can't be a "turn key" solution because there are far too many variables. If it was set and forget then it would lose a great deal of functionality.
The controls viewer shows the real layout you have set, not some genric overlay based on mame defaults. If you remap some keys for a particular game (in ctrlr files of course) those changed will be reflected on the layout and thus a lot of info needs to be gathered about your particular mame setup.
I really appreciate your feedback though.
I won't bother to update the docs until much later when the viewer is made public and new ctrlr format is supported though as virtually every part of the backend will need to be changed.