2. If you can get an offer學習一年後expungement without pleading guilty, that means your case will be dimissed. Then no worries.
3. It is suggested do not expunge the criminal record, at least not too early, because later when you want to get some official proof from the court, e.g. some additional request from immigration service, you will not be able to get it any more.
4. There is no requirements for PR application that no criminal record for the past 5 years. Citizenship requires 7 years of good behavior.