[Faceman]

As discussed in previous topics Malchor Harpell can't be killed as he is wearing Imoen's belt, which prevents him from losing his very last hitpoint. Only thing possible is to use the Nishruu bug in earlier versions, which lets the Nishruu eat up even permanent magical items to remove the belt and kill him then.
The being stuck in a dialogue can also be a result of Harpell's appearing but no talking. It occured to me, that when I was neutralizing him (protecting myself against imprisonement - he does no further attacks and only follows me without attack) I couldn't decide for dialogue options anymore. Instead the dialogue would instantly finish with a rather random choice. This could also happen if you Ctrl+Q him to his leave. Best thing is not to mess around with the Drow Rangers stuff, it's not that good anyway.