I will try and answer your questions 404. Yes the mock viva was useful. It was not so much about trying to predict what was asked but more about trying to provide a good enough, detailed answer at an appropriate level. I was worried I could not do this!
My supervisor did not attend. We decided that this was the best thing as I would be worried by him attending and he would hate not being able to speak! The examiners called him in to hear the decision and necessary changes etc.
There were some questions I could not answer straight away. I just said that I needed a minute to think about it and this seemed acceptable. I still felt pressure to start talking.
There were questions I did not expect and certainly things that I thought they would raise and they did not. Most of these were subject specific though. I don't think you can prepare for every question. I thought they would arrive with post it notes all over my thesis. In fact, the only copy with notes was mine!