CCM overall seems comparatively much easier than DDD and fire sea in the 120 star route, so why don't runners do it as the last stage before upstairs? From my (imprecise) estimations this only seems to be at most 1 second slower than the current route, but gets two of the hardest stages out of the way significantly quicker.

The second timesave is worth it, you’re in almost an hour already. How does an extra 5 minutes before going to DDD really matter

It's essentially an IRL timesave. Get the harder stages out of the way and if for some reason you do mess up enough for a reset, then you didn't waste the 6-7m CCM would've taken you.