Good points. But I think it’s easier to find a good DC than an OC. That certainly seems to be the case this year with the HCs that have been fired. But you’re absolutely right, the first priority is to find a good HC.
i think it's about interviewing a candidate, hearing him and convinced about his approach and plans on building a team.
Not just schemes and player personnel suggestions, but how he can get his players, coaches, and even front office to buy into what he is
planning, what kind of character he is, what kind of coach he is, what he sees fit to run a football team, how to adjust once the going gets tough. .
How he will he decide to delegate duties, authority and tasks to his coaching staff. How much to allow and trust his own asst's to handle tough challenges, and when
to intervene and influence his own suggestions on the fly.
Convince the interviewer that he can be a leader of men.
And the interviewer(s) have to have a knack to judge/make a decision of a coach regardless of how soft spoken or vocally loud he may be.
Just as a candidate coach has to deliver a message and plans enough to convince an interviewer that there's no question he is the man for the job.