What, if a man (NOT suitcase man, because age and description don't match) has been living with a (shocking) secret for some time? He is the perp or he knows the perp (maybe a close relative). In both cases it would be critical, if something would happen that leads police to the relative or him (maybe first relative, then immediately him). If there is an exact time of starting the police's interest, then he (man with secret) very promptly has to make disappear the suitcase. The man has another important mission at the same time: to drive from A to B because of special reason. This urgent trip he uses for dumping the case out of his car at some place on his route, near Wynarka. Maybe he crosses the highway (on foot with the case) to fool police on the direction of travel and then continues with his speedy tour.
If this man is not the perp but the other one is: he is not deceased, not before and not later, but sitting in jail a few days later. Our man drives back from B to A in a hurry sometime and later on more or less is awaiting a nice visit of police (what happens indeed).
The suitcase man doesn't fit in my scenario. If at all a suitcase man is involved, then he would have to be perhaps another relative. Maybe this older relative heard of a drama happening, heard of the suitcase dumping and was concerned about the dumping place. Maybe he wanted to help to find another dumping place. But I think somehow it's unlikely and senseless.
Question remains: where is the mother/father/carer of the little girl?