Thematically, I like it.
From what I understand of Christian mythology, no one, not even Satan himself is beyond redemption. Seems to think that a Denarian would be the same way.
However I don't think the Denarians were the casual Satan supporters, I believe it is mentioned that they were all members of the army, and high ranking ones at that
I still agree that even they can be redeemed, but I would not qualify any of them in the Innocent Bystander category. However, there is an issue of power level.
DFRPG makes it quite clear that mortals have CHOICE and monsters have their NATURE, and that having more POWER makes you a MONSTER.
Denarians have ALOT of power and therefor are monsters and are therefore driven by their nature. Any Denarian who would want to switch should likely give up ALOT of power, if only because they no longer will be drawing upon Hellfire, and likely no angelic force is gonna let them back into the pool of soulfire just yet.
Also, if you had a theoretical "good" Denarian with all it's powers it would essentially destroy the entire purpose of the Denarian metaphor. Just like the ONE RING, it's a metaphor for what having great power does to the human mind. It subverts it and changes it to it's whim, if the all powerful coin your character keeps relying on ISN'T slowly corrupting his mind, then you have given him a super weapon for free.
I guess I am asking, why are you putting a good Denarian in a game? Is it for an interesting story idea about redemption and repentance? Good, I'd say tone down the power and makes it like a Bob The Skull kind of thing. Is it to get around the drawback of having full Denarian power? Then I think it's lame!
One final note,
Lash and Nicodemus both tell Harry that they could have him do good things in their service.
People have suggested they are lying, I believe they are telling the truth. Harry knows it too, but Harry sees down the line, It's a TRAP! He will rely on the power and then the price goes up steeply and the fallen has a little more control. And you won't be able to TRULY know you can trust them until it's too late and they all ready control you. They are drug dealers! The first hit is free, it's the following ones that you REALLY have to pay for!