It doesn't bear thinking about. An injury to an arm is one thing, but having your mouth and jaw shattered must have been absolutely terrifying and traumatic. I'm surprised he managed to do anything after that.
I know kaldin, he must have been so traumatised and disorientated, it's likely he was close to collapse which is why I would guess he was easy to over power once he got to the kitchen.
Whether Sheila or Jeremy was the killer I don't believe there would have been much of a fight.
If Nevill was fit and strong he could have over powered either of them imo. He was so much taller than either of them with a long reach but in such an injured state and losing blood he must have been very weak and at a huge disadvantage.