You sound pretty clued up, mate. Examiners can tell the difference between nerves and inability and your examiner will do his best to ensure that a pleasant rapport is established before you start.
Best advice I can give on the flying bit: if you think you've made a mistake, put it behind you. Examiners have discretion to repeat certain manoeuvres and few things are more impressive than a person's ability to focus on the job in hand and not dwell on mistakes until after landing. Besides, you might not have actually cocked it up as badly as you think, if at all.
C152's ailerons are indeed differential, but also of the frise design
. Also the phrase 'compass card' can be taken to mean that little bit of paper next to the magnetic compass with the deviation figures on it. I think what you are referring to is normally called the DI or Directional indicator.