For MRCS Part A, the best time to prepare and appear in exam in my view is 2nd year of PG, because you have had a good practical exposure to most clinical situations which are asked in the exams by then. This will make reading theory and attempting questions more interesting.

However if you think you have enough time available after your pg entrance exams, and you think you are comfortable with basic knowledge of  supespecialties like urology, vascular surgery, orthopaedics, neurosurgery etc, you can go ahead with MRCS Part A.

For MRCS Part B, the best time to prepare is 3rd year, you are very strong clinically by then and you will be able to prepare with minimum effort. Your probability to pass the exam will also be higher. In addition to it, it will also help in preparation of final year general surgery clinicals.

After passing part A, you are required to pass Part B of the examination within seven years. Otherwise successful MRCS Part A result will be considered as expired.

5) Is MRCS Exam mandatory for doing FRCS?

There can be a direct entry to FRCS too but for experienced surgeons only and that too after satisfying a lot of other criteria. But MRCS makes it easier to get GMC registration, Royal college sponsorship, fellowships and jobs in the UK. 

6) Can I start my preparation for MRCS immediately after finishing my housejob?

You can start your preparation for MRCS Part A immediately after finishing your housejob but it is always better to have some basic surgical experience before you appear for MRCS Part B. 

7) What are the Subjects required to be read for MRCS Part A?/  What is the Syllabus of MRCS Part A?

The syllabus is divided into 10 modules:

Module 1: basic science knowledge relevant to surgical practice 
                 (anatomy, physio, biochem, path, micro,)
Module 2: common surgical conditions (including general surgery and surgical subspecialty related conditions)
Module 3: basic surgical skills
Module 4: the assessment and management of the surgical patient
Module 5: perioperative care of the surgical patient
Module 6: assessment and early treatment of the patient with trauma
Module 7: surgical care of the paediatric patient
Module 8: management of the dying patient
Module 9: organ and tissue transplantation
Module 10: professional behaviour and leadership skills

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Click below to read more about MRCS

https://worldsurgeryforum.net/2017/07/advantages-and-disadvantages-of-MRCS.html

MRCS Part A Preparation Plan and Books for MRCS part A