I did some searching on the topic and I thought I had found the solution - an unupdated version of the Gavin NPC mod. So I update that (I think this is where I went wrong - the uninstallation took a really long time as did the reinstallation of the updated version, a few hours each) and boot up the game.
The only problem is that now the game takes a good 5 minutes to load once I click on my shortcut to bgmain (all while bgmain.exe is active in my task manager) and when I'm inside the game it is really really slow and choppy. And it didn't fix my problem! I still have the same dialog problem. So I just decided to just kill the demon lord and make a run for the exit. A pretty easy 19,500 XP as he didn't even put up a fight.
As a sidenote on my way out I was stopped by a duergar to talk to the being in the tank on the first platform by the merchants and asked to kill this NPC and take their brain, so I did that and it turned all the inhabitants of Ust Natha against me. I couldn't even go back to turn in the brain. Is that supposed to happen?
So I know I messed up somewhere, but I don't know where and I don't know how severely. I have a feeling I've messed things up pretty bad...
I'm attaching my Weidu.log file which I haven't done anything with since I updated the mod, should I update it somehow and then repost it? Is there anything else I should attach as well?
Thanks for your support.