I am also a mechanical engineer, graduated in December 2016.
1. You can prepare CSS besides engineering. But you are currently studying MS and then PHD, which require a lot of research work. So it is up to you how to manage both together.
2. After PHD you will get a good job and comfort. A good air conditioned office and some lectures to attend. But a CSP's life is different. A CSP can do those things, a common man thinks in his mind only. It will provide you authority to do better for masses. Every group has its own routine and work and requires different mental level etc. For example, PAS, PSP and FSP are totally different jobs. Each person will find himself fit in different groups.
3. It depends upon you. If you are not satisfied with your current life, you can avail the chance of CSS.
4. The optional subjects are selected according to your educational background, your taste and age-old scoring trend.
My optional Subjects are:
International Relations
International Law
European History
Geography.
Environmental Science.