Hi Moomin, first of all, I also feel like I am absolutely going to fail, so there are at least two of us! Some people on the board have had horrible experiences with vivas, but in my personal experience I have only known one person in my field who failed outright. They appealed and were passed.
I can't help with the other question as my viva has yet to happen, but I can empathise. Do remember though - even if your examiners are experts in your subject you have spent 3-4 solid years in the literature and they probably havent. Don't panic, good luck!