1. Hellebore, some (probably poisonous, but it's made-up so it's hard to tell) mushroom, and the potion itself has a level of toxicity that makes it shitty for normal humans to drink! Lore is pretty thin -- the most detailed description I can find is: In the distant past, mages working with witchers developed this potion specifically for use in fighting cemetery and crypt dwellers that drink the blood or eat the innards of their still living victims. Witchers use Black Blood unwillingly because the potion only works when a monster begins to feast on their body. Their transmuted blood proves to be a deadly drink.

2. Lore on the oil is thin and looking the actual components are unhelpful because they're made up... SO I'd say what he would be doing, is use a chemical to break silver down into particles, suspended in oil/fat, and wipe the concoction onto weapons. Obviously, it would have to be reapplied to be effective.

I hope that provides some clarity, but the rest of that sounds good!