The story is honestly better if she dies and Kagha is turned against the dark druids.
There does need to be a low DC check to convince Arabella to simply go to jail.
The story would be better without dark druids. All of the citations referring to them on the FR wiki are BG2 and BG3 self-references, so Abits was right about them being made up by Bioware.
It's been a while since I've played the original games, but I don't recall any talk about shadow druids being made up by Bioware. Searching for "shadow circle" does bring up references outside of the BG games, but I don't play pnp so no idea what is official material or not.