"Still wouldn't have expected him to collapse unconscious within seconds with those injuries though" haven't listened to the episode yet, but I would imagine that it's possible that rob may have passed out initially perhaps from shock/blood loss/drop in pressure etc and then slipped into further unconsciousness? Rather than having just fallen unconscious iyswim.
The thing is, I don't think that Helen was in the right having stabbed him. While one might be able to argue that she should be able to claim temporary insanity or some such,it couldn't, IMO, be argued that she felt her life was at risk. Not downplaying emotional abuse here at all, but in physical terms we're talking a slap vs a stabbing. There is no comparison really.
And this is where the archers has again failed IMO. This doesn't set a positive scene for women leaving abusive relationships, if I were on a jury where an individual had been abused emotionally and had then stabbed their partner in the middle of a row, I would probably have to find them guilty.