@KamiKira00 the air duke is a bit on the stupid side but the land duke is actually really smart. if you look at what he says you can see his resolution to change the country. he is a warrior and he wishes to change the country. everyone says that he loves the country and always worked hard to defend it.
if you look at it in that way you can see an old loyal noble biting the bullet to gather all the corruption of a kingdom into one group ready to be purged by the new ruler who just came to be. hence leading to soumas plan to be cruel.
as for the songstress she is not a spy anymore as she is loyal to the duke as well as the king. she was a spy at first but then declared loyalty to the king hence the letter the duke of navy got. why wouldn't he trust her after she said i'm a spy and proved it. telling someone your a person sent to spy on them is basically giving someone the justification to end you.
as for the neighboring country that stuff doesn't matter when the populace is running on their anger that was fanned by their royal family for years. they think that all of their suffering was because of the kingdom that took their land leading to them jumping at the chance to take it back. they think that if they get the land back then they will be abe to prosper once again. their anger is whats starting the war.
i think the depth of the series is pretty good if you look deep into it.
also i don't know which duke subordinates your talking about but if its the land duke then they are all corrupt nobles who think they are great when they are just incompetent fools.