Pickpocket a potion from one guy and give it to another guy. You can save all three! But the developers intended for you to save two and ... let one die. You can also use some cheat codes to get about 255 potions, but ... nah. <img src="/ubbthreads/images/graemlins/shhh.gif" alt="" />
The Duke's killer quest is really long. I don't remember all the sequence now, but you have to go to Verdisidis and talk to one of the Sirs, and then do all of his quests, and then go back to talk to the noble, and then talk to the main merchant in the merchant's guild, and then arrange a meeting with Cybu... kill Cybu, grab the letter he drops, and then report back to the lieutenant. Don't forget to tell the dead duke, too.