Everyone is fixated on the idea Caleb was poisoned by food or drink, but it isn't necessarily so. Some organomercury compounds absorb readily through the skin or he could have inhaled it. The latter could have been a dosed cigar or a candle.
Can't help it. I've been fiddling with deadly chemicals and explosives since I was a child.
No, he's not. He'd be even more sensitive to silver than a human.
The amount of silver nitrate used would have to be small. Silver nitrate is corrosive, plus silver compound exposure causes the skin to turn blue gray when you get to a dose of 3.8g. It would be hard to miss.
Different story universe, different rules. In that one healing used up life force of the healer. Here Asa is channeling the power of light to heal. It's not the same at all. His only reticence in healing has been fear of exposure.
Mercury poisoning is the source of the phrase, Mad as a hatter. Mercury was once used to shape hats. It'll make you look nuts to everyone around you.
No wonder Asa has been so skittish about sharing his ability.