In my mind, I have Allison lying on her back, on the floor (or a couch, or a bed). GBC has her pinned down, with his legs bent under him and his knees positioned around her chest area. He is much bigger than Allison, so his weight alone would have been enough to restrain her.
While GBC is holding Allison around the neck, she reaches up and, in self-defence, digs into his face with her nails. In an attempt to avoid being scratched, GBC arches his neck and bends his head backwards. However, Allison still manages to reach his face and the resultant scratches are almost vertical.
In the picture you posted, Obby, I can see a small, dark scratch closer to GBC's mouth. In my opinion, that's where Allison's thumbnail gripped onto his face, while the other fingernails created the longer, larger scratches.
Has it been confirmed if Allison was left or right handed? The scratches on GBC's face were on his right cheek, weren't they? If so, it would have been Allison's left hand that inflicted that scratches. I'm thinking that this would have been likely, if Allison was right handed, because GBC would have possibly used his weight to restrain her dominant right hand.
MOO.
With due respect, as always, I don't really buy that mark as a scratch. I remember it being discussed at length. But to me, if it is anything at all, and especially since it is almost exactly parallel to his shirt collar, it looks more like a bit of a chafe mark only. I've had them similar to that in the past, especially with a thick-ish shirt collar material such as the one he is wearing in that pic. 
