Well, the first time i tried it, i wasn't wearing the Silver Breastplate and it didn't work. maybe it was just a coincidence. what's important is that it worked the 2nd time...
about the plague victims quest, I pickpocketed 1 of the victims and cured all 3. More XP for me

...i know there are 2 potions only in Elrath's cellar. what i always thought about was that, a 3rd Cure could be found, but it would be difficult to get. but it seemed impossible, i tried everything. i even thought the wishing well could grant another Cure :P...well it would nice if there was a third cure that would be difficult to find.
PS when i cured Sir Dante's Son, I went to Sir Dante and he doesn't give any reward. What a turn off...