...keeping her save while defending the Grove is a bit of a challenge and I would love it to be an actual conscious decision by the player for a selection of reasons.
IMHO, there is currently no in-game or story-based motivation for a "good" party to spare Minthara as part of their quest to save the Grove. She really doesn't come across as a conflicted or sympathetic character at the outset. She orders the brutal torture of a captive. She designs the slaughter of the grove. She feeds goblins to her spiders. She's just evil.
Everything sympathetic we learn about her—her cruel childhood, her indoctrination to the Absolute, her torment by Orin—comes after the Grove. After her judgment in Moonrise Towers, she is abandoned and alone and vulnerable. She could have a redemption story arc in a good campaign.
But there's no story motivation at the outset to start down this route. And it's probably why recruiting her in a good campaign is so awkward to begin with. If Larian wants to give Minthara an engaging, compelling role in a good playthrough, they really need to develop substantial additional content for her. It's not just adding some new lines or reactions.
I think it would be a big job. The origin characters with ambiguous morality, such as Astarian, Shadowheart and Lae'zel, can organically grow into more good or more evil personae because they start out morally ambiguous or neutral. Minthara being flat out evil at the outset makes it very unlikely for a good Tav to spare her.