And is it possible/a good idea to do a suite of short stories (3-4) rather than one long short story?
Yes.
And what does the viva voce require?
Orally assessable task, a series of questions asked, you answer.
It requires nothing but knowledge of your own work, where you are at, where you want to be headed, future directions, aims, etc.
This all depends on how many viva voces you have (could be one, could be two - depends on your EE2 teacher), and also at what point in time you are required to be assessed, via the viva voce.
If you have one early, you can't talk about much - except where you are headed.
If you have two, one at the beginning, one before the trials, then you can talk about your changes in direction, improvements, what you have learnt, knowledge and experiences gained, skills gained, etc.
You just have to crap on.
"work must be related to English Advanced or English Extention 1 Course" what does it specifically mean?
I suppose it has to "loosely" relate to English - be it through the area of study or not.
I linked mine to "Changing perspective - AOS"
Do I have to choose a theme consistent with an area of study?
Don't feel restricted by it, but I think it would be good if you could, showing understanding of the outcomes, etc.
But you don't always have to "do what the syllabus says".
Be imaginative/creative, explore avenues, dare to be different.