Sheila wouldn't have been injured at that point to have enabled her to run up stairs. Only when she'd realised that she would have been closer to being " captured " would she then commit suicide as the two shots would have been in quick succession given her exertion as her finger would already have remained in firing mode.
Evidence of her being the last to die is seen quite clearly from photo's.
So your supposition is that she was pretending to be unconscious in the kitchen? That she was actually already conscious and uninjured and fully aware that she was about to be caught, with all her family dead, but only elected to shoot herself at the last moment - which ran the risk of being prevented?
Could you explain the physiology of 'firing mode' finger? And would you care to explain how double head shot suicides account for less that 4% of head shot suicides given the existence of the supposed 'firing mode' finger?
In Sheila's case, the weapon was less than an ideal one for suicide which only adds to the incredulity of your assertion.
Sure, the best vantage point for a fully able Sheila would be upstairs - this gives her the longest period of time to shoot herself AND monitor progress of the Firearms Unit - as opposed to the kitchen which offers severely limited scope for observation?
Let us not forget, that the Police had been observing for a number of hours, suggesting the other 4 people in the house were already dead, but she locates herself in the kitchen, pretends to be dead, then runs upstairs without detection.
Well, it's a theory, I'll give you that.