I mean, he isn't wrong. Maybell IS a black-hearted priestess. But she's their black-hearted priestess. Sounds like neither are joining them though. At least now Elma and Luce have two witnesses to help vouch for the horrors in the dungeon.
Kalos is suddenly suspicious. I don't remember her ever mentioning the marquis family before, and now she's pointing fingers at them and the guild. So it's her word vs the guild master. Elma knows Harlein is unlikely to be involved. Also, Kalos organized this raid. It would make sense if she was the one who fed adventurers to the centipede boss through a previous raid like this. She could easily be the one who planted that miasma generating stone in the dungeon.