Insofar as FURL's, I just figured out why some pages aren't appended to .html.
In the system settings, the container suffix is defaulted to forward slash, which sets any container to that suffix. If you want the container page url to append to .html instead of / (i.e.
http://yoursite.com/container/), remove that slash.
Reverting to the home page is better than 501 errors front and back end. That means, it just can't locate the page. Look at your .htaccess file.
Also, just to be sure... when you change a site setting, are you making sure there's a red tick before navigating away? I also clear the cache if I'm concerned about a setting taking hold. I'm not sure when it's needed for site settings but I play cautious.