I can understand your thinking..however, for me, the forensics tell a different story, and I don't give a blankity blank what the DT says, dr. Horn didn't lie, or "change his story." The fact that he made an off hand comment PRIOR to doing/completing and writing up his report dose not equate to a change in "his story."
The way I envision it, he was stabbed in the heart, possibly the other wounds to his chest, and was stunned into disbelief, shock. once she began attacking him from behind his ability to fight back was reduced. Moreover, it seems evident his automatic response was flight rather than fight and given that it all happened so damn fast he never stood a chance.
Im not trying to open up the whole which came first issue...This is all IMO, MOO and all that