Bon !
Pour le probleme de kerghan, j´ai trouvé ca sur un forum anglais consacré à arcanum :
" I´m assuming you´re talking about the passage past Arronax´s cell. I´d say you need to talk to him to establish that Kerghan is the culprit and you get the quest to defeat Kerghan. You probably also need to kill the Elemental Snake which holds Arronax prisoner. ( not sure about this though.)
I´ve not tested it but I´ve seen somewhere that you need Arronax in your group to be given the proper dialogue options, so I´m not sure you can persuade Kerghan without Arronax in your group. On the other hand if you have Arronax then the guards will attack when you enter Kerghan´s castle. You need Charisma 20 and Master persuasion for sure to persuade Kerghan later on. "
-> en resumé, il faudrait avoir arronax dans son groupe , ainsi que 20 en charisme et maitre persuasion ( avec une forte intelligence bien sur ) pour soit provoquer le suicide de kerghan soit le faire renoncer à son projet !
Je verifierais si c´est bien le cas ! 