Note: It is possible to not be able to get Murmina but condition is currently unknown. (Possibly taking longer than 25 turns to get to the first fortress will cause this because I did not get Murmina afterwards even having followed the Perfect Path completely up till that point. I am assuming Murmina does not win that battle alone since the party was not there soon enough to assist her [on or around turn 30] or I just took too many turns--though this still needs verification).
((I acquired her by getting there within 20 turns, so I would guesstimate 20-25 turns is your time limit)).
I can't manage to get the event where she doesn't join. I've tried a lot of stuff. I went to turn 81 yet still manage to recruit her, so it's not a turn limit. I've waited for around 40 turns after the event in the town Lingen but that didn't do it either. I even 'conquered' a different fortress but it still didn't work. The only thing that's left that I van think of is either alignment, character requisite, or maybe even being on the true route. Tried both light and neutral alignment I think and only got Beatrice (not Dyshana, so not true route). This was done on the PC version though, so it might be only on the PC version.