im spitballing here, but since the duke suddenly showed alchemist abilities/pure mana late, aka after the og!mel died,...isnt it entirely possible he did something screwy to get those abilities from mel? like transfer the dragons "curse" to himself?
like if we considering capitalistic personality, if he had that kind of ability he would not have waited so long to make use of it and brag about it
and considering how attached he is to her bloodline, it makes no sense for him to so easily just let her be killed off unless he found a way to gain her abilities (could also have been a fake execution but idk)