I agree, I think it was Jenova that killed Shinra. But she would have done that after Cloud released her.
I'm sure the plot couldn't have continued if they'd never got out of the cell, but I'm also sure that the doors didn't open themselves.
There is no explanation as to why Jenova chose THAT time to escape. How she did it. How the door is opened.
Cloud was under Jenova's control until he got his memory back, this is unquestionable. There was noone else that Jenova could control in the building, and she'd never just escaped herself in the past. She escaped at the exact, and only time Cloud could have freed her. Cloud's cell was unlocked.
