My answers to your questions are:
1. Yes. the cheque through which you paid the money is a valid proof for a legally enforceable debt. However, you may need to back up your claim for the money and provide ample evidence (viz hand loan) to the court to show that you gave the money to your acquaintance for some purpose and the acquaintance accepted the same. Without this how can you prove that your claim is a bonafide one and against this particular acquaintance?
2. As soon as the notice is sent to the address of the acquaintance, it is to be considered as duly served and keep the acknowledgement of the notice served with you for the purpose of showing it to the court.
3. Yes. even if a third person receives such notice on behalf of the acquaintance, the same is considered as the notice having been served provided the third person adduce evidence in court of having delivered the registered notice to the said acquaintance.