By poking around in the dialogue file and experimenting, I finally reset Murdaugh's text to give me his quest. I'm not sure if it helped to have the character with the highest intelligence and/or charisma talk to him after resetting (since it wasn't working before) but the fix was very simple.
1. Bring up your cheat console window by pressing alt-tab. (Make sure cheats are enabled!)
2. Now, reopen the console, and type SetGlobal("Know_Murdaugh", "GLOBAL", 0)
3. Talk to him again, and ask him about himself, or ask why he is so articulate.
4. You should now be able to get the Ice Rose quest!