1.
Peter and John borrowed money from Gilbert, Gilbert asks them to assist a robbery because they fail to repay the loan, which earlier they agreed to do whatever Gilbert ask them to do if they fail to repay the loan. Upon succeeding, they need not to repay any further sum and will receive some diamonds as reward. There is an agreement between Peter, John and Gilbert to commit the robbery that is an illegal act, and for dishonesty, they intentionally assist the robbery, Peter is responsible for arranging a car for escaping and John is responsible for pretend to be a customer in order to divert the attention of the manager. So both Peter and John have committed the offence of Conspiracy To Rob, the agreement that Peter, John and Gilbert form and their act of assisting the robbery are actus reus. The intention of assist in the robbery and dishonesty are mens reas. Conspiracy To Robbery refers to s.159A of Crime Ordinance and s.10 of Theft Ordinance. Although John never participated in the actual robbery, he should still be committing Conspiracy To Robbery because he is in joint enterprise to commit the offence with other robbers, which each is responsible for the acts the other did, technically saying they have common design. Sze Kwan Lung & Others v HKSAR (2004) 3 HKLRD 328: foresaw as a possible incident of the execution of joint enterprise. So John is still in Principal liability of committing Conspiracy To Robbery.

Peter borrow a car form David day before the robbery, and Peter call the police 5 minute after the robbery and claim that David’s car was being stolen by others on the day before the robbery. Peter had given the police false report that the car was being stolen, which he actually borrowed it to commit other crime. Referring to s. 64 of Police Force Ordinance: any person knowingly makes or causes to be made to any police officer a false report of the commission of any offence shall be guilty of an offence. Peter knew that the car was not being stolen (mr) and he told the police the car was being stolen by someone (ar) with these elements of offence Peter could be charge with Making False Report.

2 days after the robbery, David pawned a 3-carat diamond at a pawnshop, pawnshop feeling suspicious so he reported to the police. David might commit Handling Stolen Goods, but there are too many missing fact in this case. Assume that David received the 3-carat diamond from Peter and it is stolen goods from the robbery, actus reus can be seen as there are elements of offence handling and stolen goods. The mens reas are the missing fact; knowingly (mr) we are not sure did David admit when arrested. But believingly (mr) since the doctrine of recent possession, the closest the time gap between tends to support the knowledge of David and it was two days after the robbery. Handling Stolen Goods refers to s.24 of Theft Ordinance, Peter and John have committed the offence of Handling Stolen goods as they receive diamonds from Gilbert as reward after the robbery, but if the 3-carat diamond that David pawn was given by Peter and it was stolen goods from the robbery, there will be high chance that David have committed the offence of Handling Stolen Goods.

2.
David was arrested by the police for being a suspect for handling stolen goods, after he pawned a 3-carat diamond at a pawnshop the pawnshop keeper felt suspicious. David claim that Peter owns him $100,000 for 2 months and Peter lost his car so Peter gave him the diamond as repayment. He pawn the diamond because of worrying the diamond being fake.
Normally, a reasonable man are not willing to lend his car to a person that own him $100,000 for 2 month. Secondly, a reasonable person will find it suspicious that a person that own him $100,000 and lost his car can offer him a 3-carat diamond as repayment. Thirdly, a reasonable will not pawn a diamond to a pawnshop worrying the diamond being fake, which he has the right to refuse receiving at the first place.
A sufficient defense is suppose prove there are no mens rea or actus reus, So those explanations that David claims are insufficient to defense David from the charge of handling stolen goods.

3.
Peter called the police 1 hour before the robbery and informing them of the details of the robbery but still receive diamonds in return, Peter was trying to withdraw from joint enterprise, my answer against peter in part 1 will be different as he inform the police with the details of the robbery but not false report, so the answer will be change to no making false report but not withdrew from joint enterprise. It is because although he told the police an hour ago before the robbery, Peter did not actually withdraw from the enterprises he still receive those diamonds in return. There are no deliberate departure can be seen here, so if he is really willing to withdraw from joint enterprise, there are many other way he can chose to withdraw. For example, he did not borrow the car for escape or he can simply driving away after the robbery without picking up the robbers, all he need to do is to act opposite of the common decide. So due to Peter did not have a deliberate departure so his withdraw from enterprise is insufficient, but since his report to the police are the fact, he would not be charge with making false report.