Agreed, I think the whole thing is a placebo effect. I think the forest thing that Perez said is true, but the “medicine” is just water like you said. It was a lure to make him stay in the forest and confuse him as much as possible. Or it IS actually medicine but prolonged exposure to the miasma overrides any healing effect that it has.
Either way, I think the plan was to make sure he stays in the forest long enough and let it confuse him. I’m leaning more towards the second option just because they can cover their tracks and say that it DOES have a clarifying effect but Astana was in the forest too long.
Yeah I think the « medicine » was just a placebo for the first prince to get comfortable so that he could kill many monsters and thus be drenched by their blood. He would then become delirious and reveal himself. Perez just had to wait for the adequate time. Very clever trap.
I think that special 'medicine' is just water xD and that thing about the miasma is real