To be clear, I don't think Minthara needs an arc where she changes to "good" or "neutral" using the definitions I provided earlier.

However, I think her evilness (ie selfishness) can be redirected towards good ends and that there could be a place for her in a good party, and perhaps she will even become "good" in the limited sense that she is willing to give up her life for her colleagues. (Layers!)

She could become a better kind of evil. smile

I used the example of Morrigan from Dragon Age earlier, but Morrigan's alignment never really changed. All that happened is that she revealed she's not as malicious as you initially think.