Tried to hide navigation menu on one page ended up SCRAMBLING the navigation menu on another

Are you okay with using CSS? It seems the plugin you installed has affected your header.
So I would uninstall the plugin and then with pure CSS, hide the header on the page you want to hide it.

So to hide it on the page you’re mentioning, the CSS would be:

.page-id-3262 header {
  display: none;

You can insert the CSS in appearance -> Theme Editor, and then ensure you’re in the style.css file, scroll to the bottom and insert it.