We know she had a chipped tooth. If that were caused by a blow with something hard (unlikely to be a fist) then I'm a bit surprised that there was no split lip as well.
If a blow to the front of the face caused a nosebleed, they can be the source of quite a deal of blood, and would be consistent with those photos of the Luminol tests in the car. Just one possibility. No external cut or wound, but plenty of blood.
But - another question comes to mind.... if the blood was enough to contaminate the back of the car and show up on the Luminol test - then WHERE was the injury (whatever it was) caused? I would have thought that a decent nose bleed, for example, would cause even more blood at the original site of the attack, and that no matter how carefully the clean up job was carried out, this too would show up on a Luminol test.
So, thinking logically, if that really was Allison's blood in the back of the car, then there almost certainly would have to be more of the same at the actual attack site, plus possibly drips etc if the body was carried to the car.
Of course, the other possibility is that the attack was somewhere very close to the car - maybe a blow to the face to get her into the car - alive (conscious or otherwise) then driven to either Kholo Creek, or the scout camp at Tyamolum, where she died by whatever cause was determined by the autopsy - assuming they DID find a cause.
Still a lot of unknowns - let's hope the police have a lot more knowns....