When an NPC stops the conversation to follow, you can simply leave, and click on them again right away to pick back up where you left off the conversation. This also stops them from following. You may need to wait a turn (R key) to let the AI "think" if your not in Real Time Mode.
As for Mirel, I've seen her do that once or twice before. I'd guess it's a scheduler bug, but I don't know. She's usually suppose to be in Thalassar, most of the time, so it might throw her over there.