How does one prove they can do this before being tasked to do it?
There’s no Rock of Sisyphus to roll here. Either he gets the job done or he doesn’t. But the Saints aren’t some unique outlier prone to one result or the other by virtue of existing.
They’ll hire a guy and we’ll see how adept he is at the job. This expectation of “readymade, just add New Orleans humidity” borders on absurd.
Obviously, he'll need the opportunity to prove it. But there's gotta be more to the process than just handing someone the keys and hoping for the best because, after all, he has character, and he knows how to call offensive plays.
Kellen should have clear answers to questions like, "Describe your ideal prototype for the nickel CB position? (or pick any defensive position, since Moore is an offensive coach)" "Explain the training regimen you plan to implement. How is it different from what we have, and what makes yours better?" "Describe a recent process in which you put together a plan for development/growth for a particular player under your purview?" etc.
I'm not saying these kinds of questions aren't being asked. Obviously, I'll never know. But these kinds of conversations could give some insight into whether he thinks like a playcaller or thinks like a Head Coach.