The idea that demons can't use human items is a myth.
When Malori tried to explain how she got so high level at a young age, the archer in her party tries to call her out on lying by claiming that demons can't use human items
, this is a complete myth, as we know, demons need mana to survive as much as humans do, and a huge source of the conflict between the two kingdoms, besides Fantastic Racism, is the demons' need to harvest mana as much as possible for their own survival, assuming that healing items have similar proprieties, it isn't far fetched to say that all demons can use items just fine. Healing spells seem to work just fine on demons, except when there is something specific blocking them.The Dragonslayer clan has draconic ancestry - and it wasn't consensual.
The Dragonslayers (of which includes the King, who married into royalty) need dragon blood to survive. Now the King and/or his physicians claim he needs a dragon's heart. This raises some red flags; with all the advances in Magitek and the ability to resurrect those who die from non-natural illnesses, you'd think that the king would simply consent to being autopsied a few times to research a dragon's blood substitute. The heart, in particular, implies that the necessary nutrient is magical in nature, but the level of integration with a vital organ indicates that it would be a surgical graft to replace the king's old heart, something that might prevent him from being recognized by the resurrection tree as 'human'. Unless thin-blooded half-demons are registered as human.
What if the reason Dragonslayers need dragon blood isn't because of a pact or a ritual, but because they are part-dragon? Since they live as humans in human towns, they might not be able to naturally produce specific nutrients that a dragon generates in their natural habitat, similar to how a human without sunlight requires vitamin-D supplements. This would give them the same instincts and/or senses as dragons, making tracking a dragon down easier. The heart, in particular, is simply a heart transplant.
And the only reason they forbid research into alternative medications is because it would reveal that the kingdom's beloved ruler was part-demon all along.
As for how this could happen, after generations of Dragonslayers managed to capture a few dragons at significant cost, they began... experimenting. Especially if the dragon in question transformed into a hot humanoid in an attempt to escape.
