I Am struggling with how the wrist injury could have happened? They are suggesting it was the cause of the major bleeding, but As someone has already said (apologies, it was a while back) it is quite difficult to achieve this, and usually requires a cut along the arm, rather than across the wrist. So...oo how did it happen?
The suggestion is that an artery was cut - probably the radial as it is closest to the surface (it's the pulse usually checked by HCPs). It is not difficult to cut this at all and it doesn't require a cut along the arm. Yes, if I lunged at you with a knife with the specific intention of severing only your radial artery, I'd be unlikely to hit this relatively small target. But if I'm flailing around with a knife in a panic, it wouldn't be hard to cut the radial or ulnar artery at the wrist at all.
I don't think we can assume that the 1st stab wound was all that was needed to incapacitate Rob. The lung injury that he suffered (thought to be a tension pneumothorax) wouldn't have incapacitated him straightaway, nor would an arterial bleed at the wrist. So he could still have been attacking Helen after the 1st or event the 2nd stab wound. It is possible that all 3 injuries were self-defence.