It sounds like you have a custom module that created a new frontend route. Simply removing the template and layout related to it is not enough to then be able to use the same URL as a CMS page (If you change the cms url to $url-new I bet that would work). Because now the page is loading that custom route with the styling.
You will probably have a custom module that in it's /etc/config.xml does something like
If that is all this extension does you can remove it, if it does more you'll need to remove that routers node. What that modules is you will have to figure out yourself. Hopefully it is named well.