Viva voce: Just make sure you are confident with your concept and that you know your target audience etc., and have responses for the predictable questions they'll ask you.
Concept changing: the fact that your concept is changing means that your ideas are simply evolving. Research is meant to shape and improve your concept. It is a good thing. For most of us, our initial concept was fairly narrow and pulled out of our _ss, yet at the same time we thought it was so genius. You needed that first concept though, to get to the one you have today. As with any writing, you need to get out all the crap ideas to make room for the good ones (once you realise this you also won't be afraid of writing) I don't think anyone who does a major work completes something that was exactly what they envisioned they'd produce a year ago.
How in depth: You should worry more about 'how broad'. If you pick a really broad concept such as the meaning of life you won't be able to go in depth enough, you won't be able to sufficiently explore the meaning of life in 6000 words. IF you go in markers comments you'll see that they note how people try to tackle things like existentialism and end up getting a bit lost in their narrative. It is a struggle for me to not become too tangential. As we continue to edit our major work though, the concept will become more refined.
I am reluctant to show my teachers my major work as it as crap, but I know that I need to, how are you all with the confidence aspect of it?