As mentioned the whole proces could only take a few seconds.
Jeremy was ready, psyched up, strong, fit & determined. By the time Sheila realised what may be happening the first shot has been fired.
You really have to make Sheila a raving, halfwit idiot to make your crazy scenario fit the facts. I don´t think she was.
In case Jeremy committed the murders, we have to regard Sheila as a young mother of two who would have slept with one eye open in case something happened to her children (mothers tend to do that), not a clueless halfwit! It is quite insulting to her, in my opinion.