Some Suggestions

Though Adding Mexico would be interesting, I fear to do not speak sufficient Spanish to cover it enough to give it justice. As a wiki page, hopefully a Spanish speaking student might one day want to add such content to this page and complete it. I also am unsure how to address the issue in the page currently and maintain "wiki speak". In a paper I could say "For the purpose of this essay I am focusing on English speaking countries in NA" but cannot really do that here. I am open for suggestions on how to address this however perhaps in the overview or elsewhere in the page.

I see where the difficulty is. I agree that the overview is where you should put it. Perhaps state "Late modern North American archives of Canada and the United States..." or you could have a section for Mexico and simply have a link to unfilled new page, though that's highly misleading at this point.

Having said that, to stick with Wiki format, you may want to remove the title from the "Overview" section as most Wikis tend to jump into a definition of what the page is about without it being a separate section.