I think Corgino and Ralph might not be that stupid this time. (Conditionally.)
Though they are way less smart than I assumed a little while ago, and the bar was low to begin with.
1. First, I assumed that they would try to frame Cain for something, possibly creating an international incident. Cain is pretty pacifist by nature, though he will defend himself with the strength he needs when attacked. He would likely have subjected himself to the legal system to prove his innocence. And his demigod powers would have been a lot less useful then.
But no, they decide to go for the violent approach once again. Because they can't learn.
(Also, one point that Ralph doesn't know: Cain is not just able to fight dragons. He solo-killed a great dragon at the age of 5. He is way stronger now. Corgino might know to some extent though. He saw the dragon in Cain's entrance hall and should know where it comes from. That was not kept particularly secret.)
2. Second, given what they know, Cain might still be an approachable target. Mobilizing a whole dark guild might include some S-rank equivalent (who might not be official S-rank in the adventurers guild). Possibly, this might include splitting forces to target the ladies as hostages. This is not the dark guild in some remote town like Drintre.
Since they don't know that Cain is not just S-rank, but a literal demigod and apostle of all the gods, they can assume that a whole guild might be enough to match him. This is not all that far-fetched if they do have some S-rankers in the guild. If not, their plot is not just doomed, but downright stupid.