Is the victim's (Jennifer Welles) cause of death definitively established? Is it the blow on the head that caused death or the exposure to gas?

I don't think it was properly established, but her death seems to be similar to Ric Carsini's in that the victim was rendered unconscious and then suffocated due to lack of oxygen. In Jennifer's case due to the gas.

I think she was knocked unconscious and then died from inhaling the gas fumes. Since part of her "motivation" was suicide, I believe that is what we are all made to feel.

Clearly, the head shot didn't kill her or the suicide spin goes all out the window as she wouldn't have inhaled any of the fumes.