Can't speak to trepanation. But Calomel or mercuric chloride was used as a disinfectant and also as an antibiotic, often against venereal diseases like syphilis. It was indeed toxic, ( and if exposed to light would shift to a more water-soluble toxic form, so it was kept in a dark bottle) but anecdotal evidence would suggest it was sometimes somewhat effective. James Boswell's diary, for example, describes him being treated with it, apparently successfully.

But you could say that , really, the idea of chemotherapy has just been around for a long time- take mild poison and hope the infection is killed before you are. It wasn't too far of, perhaps, to take calomel. Syphilis was found to be a fragile organism, which allowed Paul Ehrlich to come up with the first really successful treatment for it based on arsenic, called salvarsan. Some of the modern cancer treatments are perhaps not quite as deadly, certainly are better understood. They are nonetheless quite brutal, and if there's no better alternative, people will often suffer the brutality. People in the days of Heroic Medicine would do the same. And, like today, they would also often view doctors with skepticism. Moliére created a biting picture of pompous , avaricious and useless doctors in The Imaginary Invalid.