Why is this even a discussion?
The event lasts for 14 days.
In those 14 days you have to at least finish 12 missions of the 14 in total to get the mission event girl.
Once the event is over, you cannot get any of the event girls anymore, so by extension that also counts for the mission girl.
Yes the last event mission was one for 7 hours, but that just means you have to start it at least 7 hours and a minute before the end of the event.
Anyway, now you know what happens if you do not do it like this; you do get the kobans, but you don't get the girl.
It might hurt, but at least you have learned a valuable lesson.