1. I accepted a quest to clear out a den of brigands or whatever (static location quest, not follow the tracks). I did so. On the way back to collect my reward I was interrupted with a random dialog saying my band had found tracks leading off somewhere etc, I followed the tracks to another den of brigands (static location again, not wandering) and killed them as well. When I returned to the World Map the spinning gold coin indicator on my destination town was gone and when I arrived the quest did not complete. I saved/quit, came back, no joy.
2. It probably is reproducible but there is no easy way to do it, given that you need a random dialog to appear.
3-5. I forgot to do a screenshot, but the rest is attached. Thanks!