Found my own answers.
The stern was not rotating freely in the video. Here is a still image from after the explosion subsides.
But, the ship was cut in half. As it was sinking, the stern did rotate separately from the bow.
Note the orientation of the antennas on the aft superstructure.
So I guess the only question is whether the ship was immediately doomed or whether the snapping off of the stern was caused in part by the ship beginning to sink. My guess is dooooooooooommmmmeed!