Will try to answer your questions - I regularly do AS and A2 oral exams for my students.
a) as a teacher-examiner, you do not mark the exam, it is recorded and sent to the exam board to be marked. Visiting examiners however do mark the exams, but I presume from what you have said that your old school has asked you to be a teacher-examiner.
b) you don't need to know any stats at all. If a student tells you, as part of the exam, thatl you 20% of Germans take drugs before the age of X, then accept what they are saying. Never try to correct them, either for their facts or their language. They are not being tested on their factual knowledge here, but on their ability to express themselves and give opinions in the foreign language. No AQA marker will check the veractiy of the stats they are giving, so long as they sound reasonably plausible
c) be aware that you need to have prepared some follow-up questions for the stimulus card in part 1. There are 5 printed questions, but part 1 is meant to last 5 minutes, and in general the answers to the 5 printed questions last between 2 and 2 and a half minutes (depending on the student), so you need to have pre-prepared some additional follow-up questions to fill up the remaining time. These questions need to relate to the 5 printed questions, and should allow the conversation to continue from where it left off at the end of question 5
As for part 2 questions, you really need to ask your old school what the students have prepared. In theory students should not know the questions they are going to be asked, so you should be able to ask them anything on the AS topic areas, but I would try to find out from their teacher exactly what they have focused on and what their ability levels are, to allow you to tailor the question accordingly
d) They should cover all four of the topic areas (see AQA website) over the whole oral exam. One topic will be the topic on the stimulus card. The remaining three will be part of the part 2 conversation. As part of part 2, students can nominate their first topic, and choose the areas within this topic they wish to talk about. Again, you need to speak to the school to find out if the students have nominated topics. If they have, they should have a headings card, which will give 5 headings indicating the areas they wish to talk about - this will inform your questioning
Hope all that helps, can give more info if you need