No doubt there is a story connection with the witches, but if we're strictly talking about the process of getting the "Her Reason To Live" quest, I suppose it makes sense to just focus on the mandatory steps. As far as I understand it after playing through the game 3 times now, all you need is that dungeon event where, as you said, Rance learns about Ran's sadness. How to get that event is just 1.) Ran in the active party, 2.) Mill, Maria and Shizuka recruited and 3.) Lion Mind done (given in world 1 since it's not all that likely that anyone deliberately avoids the relatively easy Lion Mind [15 difficulty] while unlocking Malgrid 4 [33 difficulty] for Maria on their first run.)
Btw, I don't think the generic dungeon events (those that are listed in the data log by that name) require you to be in Malgrid to trigger. For one, I had plenty of those events trigger on story-related missions, and secondly, the few dungeon events that are conditional (like Tilde needing to be close to Kenshin's level to trigger one of the former's events) are identified as such in the data log if you turn on spoilers. In other words, with spoilers turned on, the data log would tell you if Ran's event had special conditions, which it doesn't.