Raistlin, I have a few questions.

Did the guard open the cell when it saw Jenova enter the room, or after he was brutally attacked?

If the soldier was brutally attacked first, then there should be a trail of blood from the soldier's position to the cell door. And if so, why wouldn't the soldier open the cell closest to him? Also, why would the soldier open the cell and then crawl back to the entrance of the prison? The soldier was injured and wanted help, so why would it crawl away from the people that would help him?

If the soldier saw Jenova and then opened the cell, how could have Cloud and Tifa not heard Jenova attack the guard? You would have to ASSUME that the cell door takes several minutes to open, but we known this is not true because Cloud opens the other two cells in a matter of seconds.

So, I think we can pretty much scratch out the idea that the soldier opened the cell.