I also am at the last step with needing to convince Duran of Magran/Woedica working together, and I don't have enough options to do so. I have exhausted the entirety of his dialogue tree, from what I can tell.
Here's my save file, so you can tell me if I just am failing to see what needs to be done, or if there's a bug:
https://www.dropbox.com/s/6vu3gt3j1bgs125/fdad894130824eb7b2fd03d2d0b1503d%2019428834%20Brighthollow.savegame?dl=0