Okay, first I have to admit my group and I are pretty new to the DFRPG. We're still working the kinks out of our interpretation of the rules. Now that disclaimer done, I'll get back to answering your question.
The Lawbreaker powers do appear to be very black and white. Do this and you are forever stained etc. This can seem rather limiting but as you've said in the novel canon, law breakers can seek redemption or to better themselves. I think, however, it's a bit of a toss-up about how successful they can be.
Molly walks a pretty grey line with her use of mind magic and then stepped over the line in Ghost Story. Harry killed Justin and has been a mostly good boy since, although, he has deployed potentially lethal magical force against humans. That it didn't end up lethal... well there was quite the element of luck. Also, his resolve to protect his love ones has been strongly implied to know no bounds.
Does this mean once a Lawbreaker always a Lawbreaker? That's more complicated.
Perhaps we should also consider what Lawbreaker means from a more than mechanical POV. I've heard it described in the novels and online, that the power represents the practitioner's belief they have the Right to do what they did. To mess with someone's mind, to take a life etc. I think this makes sense. Until a practitioner can truly shed that belief, not just admit they did wrong, and truly decide never to do it again, they would be Lawbreakers. Both Molly and Harry acknowledge what they did was wrong but they'd both do it anyway if they thought it justified.
For our table, removal of the Lawbreaker power means lots of RP, change of an Aspect and repayment equal to the Lawbreaker refresh with GM permission. If they break the same or a different Law, they get the old Lawbreaker power and a new one. This may mean the character gets permanently taken out as an NPC.
Of course, your mileage may differ. You get to decide if or when someone no longer deserves the Lawbreaker power.
As for 'adding on' to the Lawbreaker power, I'd strongly suggest you avoid that. To expand the power suggests that magic innately condemns actions other than law breaking. It also adds a mechanical bonus greater than what Refinement generally gives. Essentially, it's a floating 'specialization' to all types combat magic, to use your example.
Everyone would want to break laws with that kind of reward. Don't forget that the Lawbreaker power is meant to act as a mechanical penalty by removing a player's ability to tailor their character by choosing where to invest refresh, raising the risk of taking the character out and on the RP level, opening them to punishment from the Council.
If you do want to be able to facilitate a redemption arc with a character, that's fine. Just work out with the player how to do it and what rules or thresholds you want to employ. Maybe ask the player to make their Lawbreaking and search for absolution be expressed in their Aspects. Have them invoked. The Lawbreaking should be made meaningful to the character and thus player, not just a convenient bonus with minor RP consequences.