First of all, I'd suggest you go back a couple of pages in the Pianist's Corner and look at the topic 'Anyone that knows anything about the AMEB AMusA (piano)?', as we provided a lot of similar information to the OP there. But I'm happy to help you with your specific questions here too.
1. I don't have any official statistics about the pass rate- there are various rumours that float around about figures but to my knowledge the AMEB hasn't actually said. What they do say is that the pass rate is much lower than for Eighth Grade. At a conference I went to this year, I was told that the AMEB are getting a lot tougher about awarding the diploma.
2. No, that's not a definite fail by any means. Examiners do not expect perfection. Remember that this is a PERFORMING diploma, so what is very important is the ability to show performance attributes- i.e. professionalism, attention to detail, ability to develop and show a convincing and music interpretation of a work, and being able to carry on even if there is a mistake! It all really depends on the context in which a slip occurs, but confidence is the key here. If you play a wrong note or have a slight rough spot, that doesn't automatically mean they will fail you.
3. Again, no- perfection is not expected. And they may not ask about modulations at all! The important thing with General Knowledge is that it is a discussion between you and the examiners- they want to see that you can talk about the piece confidently, that you know about its historical context, form, structure, etc. Refer to the Manual of Syllabuses for more info, but a wrong answer on its own isn't going to cost you the diploma. They may comment on it- but it all depends how everything else went.
4. Playing from memory is good and examiners will often look favourably upon it, as it can add to professionalism and the performance attributes. However, if you feel that you are more than likely to slip up, it's better to use the score. The examiners will not 'disapprove'- there is no memory requirement for AMusA (unlike LMusA); you are simply encouraged to play from memory, so they may just comment on it as an area for future development. If you've tried performing from memory before and you feel it has made you more insecure, then that's a signal that it's better not to this time round- but a great area to explore further at another time!
If you got a C for Eighth Grade, this doesn’t mean you will fail AMusA if you’re prepared enough. However, be aware that the standard is much higher and the AMEB recommends you have passed Grade 8 with a B or above if you want to do the diploma. Avoid too much last-minute practice as this will only make you even more nervous; just stay calm and remember that perfection is not expected- they want to see how musically you can play in a proper performance situation. If you do not pass, that is not the end by any means- you can always try again and there is now the new Certificate of Performance for those who are not quite ready for AMusA, if you want to go back and do that. Have you managed to perform your program through for others? This can really help with nerves and give you a sense of what the exam might be like. And what are you playing?
All the very best for your exam and do let us know how you get on!
P.S. It's always better to have a page-turner who can read music!