Physics Spl

Yes, you can go through JAM. Take 1 year preparation for the same since you are not a science student, you have to give time to it. If you are from civil, electronics or any physics related branch then it will be more helpful for you.

First, go for the basics of physics and try to understand the unit system then it will be an easy task for you to score in the physics exam.

No, for PhD in a particular subject you should have the PG in the same subject.

He has many options. If he do PMCS after PUC, he can work in Software Company or else he can be able to become a lecturer in engineering college also.

Please use the link provided below for a detailed list of entrance exams which you can pursue to start a career in physics. I have also mentioned some of the more popular scholarships which you can opt for if you qualify for the same.
https://www.shiksha.com/science/articles/top-bsc-msc-entrance-exams-in-india-blogId-13484
https://www.shiksha.com/science/articles/popular-bsc-msc-scholarships-in-india-blogId-13494

To become an astronaut, you have to get these basic requirements-
Bachelor's degree from an accredited institution in engineering, biological science, physical science, or mathematics.
At least 1,000 hours pilot-in-command time in jet aircraft. Flight test experience is highly desirable.
Ability to pass a NASA space physical which is similar to a military or civilian flight physical and includes the following specific standards -
Distant visual acuity- 20/100 or better uncorrected, correctable to 20/20 each eye.
Blood pressure- 140/90 measured in a sitting position.
Height between 62 and 75 inches.