There's still something they can do with her. When it comes to struggling with the ghosts of your past, when you overcome one aspect of it, another pops up. It's like peeling apart an onion layer by layer unless you get closer to the core of it.
Here's an idea. While her guilt may be solved, her fear may yet remain. Fear of what, you may ask? Fear of NMM repeating again. In all of the episodes showed her be herself, you can tell that she has a passionate character. When she's alone, time and again she might ask herself "what went wrong?". She may not be NMM and she does understand that, but the real question is "how did she allow herself to be corrupted?" and "Can it happen again?". Her fear can stem from feeling vulnerable or susceptible to repeat the same mistakes, especially when putting the stakes and the consequences into perspective. After the Tantibus happened, she'll want to be more wary about how she decides to keep herself in check. That whole affair could actually have made her fears worse as she put all of Equestria in danger for not keeping her creation under control while all she wanted to do was keep herself in check. Is she really nothing more than an incompetent fool where everyone including herself is better off not having any power? That'd be a very uncomfortable question to ask herself. All said, perhaps she really should be afraid of being a danger to every pony in Equestria.
This leads to the actual lesson. When terrible mistakes are made, it can make us afraid of repeating them in any way, shape or form. Thinking about it, this fear very well came to life with the Tantibus as she, by her own doing even if unwittingly, put all of Equestria in danger. That would be one reason for her to believe that she truly is vulnerable or susceptible to creating such big messes. Paradoxically, by accepting that she will always be vulnerable to making mistakes, not only will it get her to stop going to extreme lengths to keep herself in check, it will also dispel these fears altogether. To make herself invulnerable to making any kind of mistake is asking for perfection. Instead of trying to be perfect, even if that's in a very specific area, she must instead get to know herself and be satisfied with herself for just being more prepared than she ever was to not repeat the same mistakes -- not totally immune, just better prepared. That's all there needs to be to it.
Yeah, that sounds pretty hefty, doesn't it?