What makes most sense, using occam's razor?
IMO it makes sense that N descended the stairs by herself for whatever reason, to use the toilet or see her mum. Being removed by a 3rd party from the mezzanine floor doesn't seem likely.
Once she reached the ground floor, it would make most sense for her to seek her mother or use the toilet, but something has happened. If she decided to leave the house, it would make most sense for her to use the door, but we are told that forensically there is evidence that she left via the window.
If there was a 3rd party, it has to be opportunistic, as the timescale makes no sense for a planned kidnap/grooming. If there was a 3rd party prowling, it it surprising that they made no noise to rouse anyone else, I.e. the ground floor occupants.
What if there was a prowler or burglar and this person was surprised by N coming down in the night? Again, it's very surprising that there was no sound, and strange that N apparently left via a window. What if the burglar came in through the window, and N for whatever reason saw an escape via that window as the other door was blocked? Again, surprising that there don't seem to be signs of disarray if that is the case.
For whatever reason, N left the house whether with someone else or alone. From the descriptions given, it does sound unlikely that she was confident and capable of doing that under her own steam. So this aspect is mysterious. Whatever happened, she determined that being outdoors was preferable to being in the house.
Why is there no trace? I think 3 possibilities:
*jungle too thick - she has been missed
*driven away by a 3rd party and no longer in the area
*fallen into a storm drain or water
All JMO. It is a very confusing case. I really hope and pray there are some answers because a person cannot just vanish.