So the death penalty is off limits for punishing the invaders... Maybe a life-sentence in a far less luxurious prison than the Ivory Tower or the Adalgisa Villa might be fitting, along with taking a line out of Ehrenfest's treatment of Veronica and Bindewald and regularly drain their mana and use it for the good of Yurgenschmidt.
As Detlinde, I'd lock her into a room with Hartmut for a few hours, with only the instruction that she absolutely has to be kept alive and make sure to never inquite about the details of what happened in that room.
That being said, if I was the next Zent, I would welcome those Lanzenavians who are still in their country, but wish to immigrate, into the Yurgenschmidt with open arms. With the still acute mana shortage, those people would be valuable additions to the kingdom. They might even bring with them some knowledge of techologies, not present in Yurgenschmidt. I doubt that it's possible to replicate the silver material in a land abundand with mana, but they might have some other stuff that can be used there. To fit into Yurgenschmidt's nobility, they could give them an accelerated academy course, somewhat similar to what the ex-blue priests got, upon their return to noble society, albeit with more courses about Yurgenschmidt etiquette.
The question is whether it'd be better to split them up and scatter them accross multiple duchies, or make a new duchy just for them, on the territory of one of the disbanded duchies. And whether to give them noble status according to their mana, or start them all out as laynobles with the usual three generation rules to go up in status. Of course the latter can only work, if they are split up. An entire duchy of only laynobles would probably not work very well.