Not everything has been revealed yet and this arc has been going on for literal years so I may not recall everything right, but here's my best approximation.
The organization is trafficking people. Every time anyone gets close to catching them by the tail, they cut it off and the investigation goes nowhere. This is because A) everyone doing the actual trafficking is expendable or if they have too much knowledge of the organization, they're killed the moment they fail, B) a good chunk of adventurers and law enforcement are in on it and can sabotage investigations without arousing suspicion because they all have history with everyone around them, C) the nobility behind the organization are covering up traces of the paperwork and logistics that would be evidence, and D) when they do get evidence, it's getting destroyed by traitorous law enforcement.
As for the current situation, 1) Ivy is the next target, and she knows that because of her slime that can accurately read people's good-evil alignment at a glance. Thanks to that, they have a means of uncovering all of the traitors, so that takes care of B and prevents D. 2) Ivy has plans to meet Mira soon where it's likely Mira and the org will try to kidnap her, so that's their best chance to lay their own trap before the org catches on that they know Mira is a traitor. 3) Ivy deduced that Mira knows too much, so because of A someone must be watching her to kill her and erase all traces of the org if she fails and gets captured.
4) They already managed to capture a hideout, and thanks to Ivy, they managed to hide the money and other evidence they found from the traitors as they moved it to a storehouse/base, preventing D. 5) They've discovered that the org is harboring violent criminals in the forest, most likely to have them ransack the storehouse/base to destroy the evidence and recover the money while appearing on the surface as simple bandits. Once word gets out that bandits are in the forest though, the guild will have to organize adventurers to go kill or capture them, but because of B it would fail if they let things play out normally and traitors were mixed in with the subjugation force.
6) Lastly, the feudal lord Forolda has his own investigation into the org and has been tricked by count Faltoria who recently recommended him a bunch of adventurer's to guard him, likely with plans to kill him and the adventurers all at once and install a puppet feudal lord while everyone's too busy grieving.
So with aaaaaaall of that established, basically the plan is to capture Mira when she tries to capture Ivy and protect her from the org assassin while simultaneously attacking the bandits, making it seem like the org's plot to attack the base has been leaked and forcing them to attack ASAP, before they're ready. By securing Mira and protecting her from the assassination, they secure a direct witness with enough direct experience to incriminate as many people as possible. By wrapping up everyone in the bandit subjugation, both the adventurer's guild and the vigilante corp including all of the traitors littered throughout both groups, they're creating a chaotic situation where the traitors can't move and they can't secretly communicate and conspire.
That covers everything but 6, but that's probably not an immediate concern as once they have Mira and the evidence secured at 4, they have a strong enough case to convince lord Forolda to have count Faltoria arrested.