404 Override Not Working

If the need should arise to call the show_404() method from within the custom Notfound Controller that would cause an infinite redirect loop. Meaning if he would need to call it he would have to again build something to overcome that issue.

This would be true if they did need to call the show_404() method within the Notfound Controller, which isn't a custom controller, they're extending the standard CI_Controller. From what they posted it does not seem like they would need to do this though as they were trying to set one controller for all 404 pages via $route['404_override'] = 'notfound'; which would suggest they only needed one 404 page, if they did then it's a different problem and a different solution is needed.

Also any 404 unrelated to the forum he is building, will not be routed to the custom notfound controller

What if they only need one 404 page for their application? They didn't specify they needed a specific 404 page for their forum and a different one for other parts of their application, if this was needed though they could make use of the $page argument and redirect to the desired 404 page with a small modification to the code if it was required, again different problem, different solution.

You are “fixing something” that is not broken. How is that a correct approach to anything?

I don't see how this is "fixing something that is not broken", it's changing how something works so it suits your specific needs.