What happens if you put the fever fetish on a commander in the same province as another commander who holds the chalice? (or when you have a healer healing or the gift of health up?)
If memory serves, as long as the fetish is equipped the disease continues uninterrupted. If you have healers or chalice though, easy solution is to unequip the fetish or trade to another commander before death. Once theyre not holding it they can be cured normally.
iirc the chance that the fever fetish generates a gem depends on the amount of hp left compared to max hp. 1-50% hp left 100% gem, 51%-75% hp, 50% gem, 76%+ 25%. But I could not find a source for this information.