@max_appian: - Spoiler Alert -

As to the reason why silver was in the brazier, there is a hint somewhere in the archeology-related documents that romans used silver to avert evil. Also when you ask Garrow about the silver, he says something along the lines of "it did not work", so therefore Garrow may have used the silver against the "evil" in his hallucinations.

I am also perplexed about the issue of the key - how it got in the bath hall if Sir Gregory is not the murderer and it is known that Sir Gregory gave the key to the servant (thus he knew where to look for it).