IIT

Colleges accepting IIT JAM 2018 scores/ranks for admission are mentioned at the link provided below. Candidates are informed of the total number of seats available for different M.Sc courses offered at all participating institutes. A total of 15 IITs, 16 NITs, and two Centrally Funded Technical Institutes (CFTIs) accept JAM scores for M.Sc admissions:
https://www.shiksha.com/science/articles/colleges-accepting-iit-jam-scores-ranks-for-admission-blogId-13971

Yes, he can get NITs but not any of the IITs.

It is less likely.

You can get IIST if your JEE Mains rank is good. You have low chances for IISER.

It's your choice if you balance between your school study and JEE preparation then you can start preparing for it in class 10th.

Resonance is the best coaching institute for IIT followed by Allen and Bansal classes.

There are so many scholarships available after class 12th
Like
1. Foundation for excellence
2. Ongc scholarship
3. Seimens scholarship
4. IOCL scholarship
5. FAEA BHEL scholarship
and many more. You should visit the website buddyforstudy. Com to get the list of some useful scholarship after 12th.

If you want to teach the students interested in the preparation of IIT there is no restriction that you have to choose B.Tech or B.Sc. You can do either one of them. There are many B.Tech holders who are even pursuing M.Tech and becoming lecturers. So it's up to you in the field you are interested. technical or non-technical.

No. The answer is obviously a no. You'll have to start from the very starting.

There is only one criterion of minimum 40% marks in each subject. You don't need to worry about it.