Eugene agreed so his family was helping him.
His family knew what they were doing though and never had Eugene or his people stay at their official home.
The rebels were always housed in properties not written in their name but of their other comrades.
It was why Ferdinand and the knights had caught some nobles whom they thought were the ones who led the rebellion but were just part of the group.
Some were also made to believe they were truly leading it not knowing they were only manipulated to believe so. Thus, their confession seemed honest.
As for the reason why Eugene specifically targeted the former villagers from earlier as his temporary men, it was because he planned to continue mining that location.
Arabella learned from Eugene and his close associates that it was Eugene who had the area mined when a landslide in one of the areas showed signs of several ore deposits.
Eugene's family owned two mines here in Sibruh and it was one of their sources of income.