As satisfying as it is finding the little treasures scattered around the map on my own, I don't think I would have been able to find all of Duinlan and Hythe's urns to finish this quest. Not without running around the map mindlessly for hours... But the thought of Duinlan not being reunited with her love broke my heart, so I looked for a little help and I found this detailed guide (with screencaps and map locations). I thought I'd share it in case it's useful for other players in the future:

https://steamcommunity.com/sharedfiles/filedetails/?id=1332100855

I found 8/10 on my own, not bad I'd say. In some locations the Red Mist, for some reason, didn't highlight them (perhaps I needed to be closer?). The reward was certainly better than I imagined. Now things are getting really heated up in the game, I can't wait to see the conclusion!