The empire only evidence is a magic cannon that was sold to a foreign country and then brought back and another noble brought it to them, they can't use just that as evidence of treason as it could be a plan to frame MC father, they will likely keep obeervers around and then make them leave and plant spies.
s even possible they will not remove the public agents and the duke will still restart production because for some reason he needs that money.
Is a pity, unlike other "defective" stories this MC family didn't abuse or exile him to die, they simply gave up on him, and his father even regretted his own words, wonder what is his reason for risking his position and the country safety for some money, I don't feel tht is simple greed.