I think you're selling them short. Yes, there are likely to be a couple who are 'green recruits' and a far from their potential. But others have been alive for considerably longer, and they all share at least one strength: an ever-present bank of knowledge in the form of an ageless fallen angel. And this being has personally trained/indoctrinated them 24 hours per day for as long as they've had their coin, which in most cases will have been decades if not centuries. With a handful of exceptions, they are likely considerably more powerful than the 12 or so refresh you grant them, which is barely more powerful than a young Harry Dresden early in the series.
I think adding the Demonic Co-pilot makes a good amount of sense, and it kind of surprises me that all of the stat-blocks don't have that. I disagree with beast change; I think that Denarians probably do not sacrifice any of their capabilities when they look human. Instead, their 'beast form' is more a personal preference and source of intimidation than anything else. So Human Guise fits better.
In addition, wasn't it mentioned in one of the books that all of the Denarians were spellcasters, though some were very weak due perhaps to disinterest and lack of practice? (I.e., possibly low ratings in Discipline and/or Conviction.) If so, this would probably indicate a minimum of Channeling and Hellfire for each, though these would be abilities not used by some of them.