IIT Madras - Indian Institute of Technology

Well, all these are top institutions for engineering and other research courses. But if you want to choose one of them, then it should be depending on your interest. You need to choose the branch as per your interest and the one that will make you more comfortable to achieve your future goal.
If the field doesn't matter, then go for Aerospace Engineering from IIST from where your chances to go in ISRO would be increased.
All the best.

Depends on how quick you want placement. Almost all IITs now have nearly the same placement scale. So don't overthink about the choice.

No, admission in IITs is purely done on the basis of rank scored in JEE Advanced exam. One has to score a good rank in JEE Advanced in order to get a seat in IITs.

Firstly, dear,
It is good that you are concerning about your career. Do complete you 12th with good marks then attempt for IIT entrance exam where you can get a branch of aerospace or you can give the entrance exam for IIST to get into ISRO. You will automatically know all these things as you will grow up. All the best
Keep smiling.

Click on the link given below and watch the video till the end. After watching the video you can find the branch name that you could get in these respective colleges. https://youtu.be/hOlit6Un7l0

Friend, your rank is very good, And it's sure that you will be getting Government Colleges in Bengal. ECE Branch is good, I prefer you to go for it. It will give you placements in telecommunication companies like Reliance, Airtel etc. , online shopping industry. And many more. Go for the link below to find some Colleges on the basis of your WBJEE merit:
https://googleweblight.com/i?u=https://www.shiksha.com/b-tech/resources/wbjee-college-predictor&hl=en-IN&grqid=IryXv5Dn
Best of luck.

Both are great institutions. But if I were in your position I would chose IIT, Madras because of its ranking and teaching and much more. I don't know much about IIT, Hyderabad. Good luck.

The process for aeronautical engineering is based on JEE score in the desired institute. So if he is pursuing 12th then tell him to study hard to score well in JEE.