I was a passenger on this plane
I have not read all posts on this thread but I was a passenger on the plane and can clarify a few things first hand.
First, there was an initial loud bang followed quickly by the vibration. I imagined that a cargo door had blown off. The vibration did last the entire return flight except for the approach to landing when it all but stopped. Presumably lower airspeed reduced the windmilling force.
The vibration was fairly constant and low frequency - maybe 5 to 8 Hz I am guessing. It did seem so wane and surge. During the surging it was extremely unnerving. The toilet block behind me was wobbling so much that the noise prevented me from always hearing properly whatever was being said. In fact, during the emergency briefings from the crew they had to deliver it in person to a couple of rows at a time so we could hear them.
At times the vibration of my seat back was too much to be able to rest against. I had to sit forward in the seat.
The captain did suggest we pray on two occasions. I was not upset that he said that except that I hoped it did not mean that he was substituting reliance on a higher being over his own effort. He did sound emotional on one of his announcements when he appeared to have to stop mid sentence and then compose himself to continue. Then again it could have been something else that diverted his attention.
He did initially describe the number 1 engine as having siezed. Although I could not see it - I was sitting in the middle, intuitively I imagined that it was still spinning asymmetrically and that must be causing the vibration. I did not know why it could not be stopped but presumed the captain could not contain it.
The passengers around me were discussing why we were not landing at a closer airport. We though we must be close to Learmonth.
The cabin crew did an excellent job. You could tell they were worried but they performed well.
My gut feeling was that we had a 50:50 chance but really I had no idea. I knew the captain was in control but with all the wobbling and vibration I was worried something would break and he would lose control.
Landing was sweet. The captain stood at the door and shook the hands of every single passenger. The crew got hi 5s and the passengers were giving hugs all round.
It's good to be alive!