The old man is a colleague of the Mc's father and is peeved at being stuck in his shadow. Wanting to stand out for his own merit, he takes a risk to develop a miracle restorative medicine even if the components are poison. Thinking "Great things, require great sacrifice" or "Those hacks know nothing of it's potential. I'll show them and overcome the side effects with my skill!"
He starts by setting up a manufacturing lab in the sticks and takes advantage of the orphanage as a labor force. The director is in on it but only because the orphanage desperately needed the money. The Orphanage's director has gotten sick from exposure but can't do anything about it despite knowing the cause. The orphan's are oblivious and don't know why the director refuses the "restorative medicine".
Despite looking down on the orphans as "dirty", The Healer takes one as an assistant to help procure material and as a potential scapegoat. After being taken in, cared for, and trained in healing, she is loyal and respects her master to a fault. Who is she to doubt her master if he tells her "it is not a dangerous drug, but a restorative medicine in the hands of a skilled pharmacist." or "It is still in the development stage so it is expected that there would be a few kinks." "This is the daughter to my nemesis that stole my thunder. Keep an eye on her." "These bureaucrats don't understand my work. How dare they try to interfere before it's completion. We may have to dispose of them. Prepare the Tea."
After discovering the state of the Orphanage director, A conflict of interests arise as to her loyalty to her master and affection towards the beloved director. Only by confronting the effects to those she holds dear does the doubt start to take merit and her loyalty start to wavier.
I expect the apprentice to turncoat and the Old man to try and claim ignorance and pin everything on her.