CBSE 12th

Most of the universities offer admission on 6 band, maybe their might be some universities which will give you admission.

Chemistry comes as a basic subject of general study in your first year of course of engineering if you go for almost all engineering courses. But, there isn't much to worry about its just the basic things you have learned in your 11th and 12th that's just freshened up with a little more of knowledge which is never too much to care about. So, if you've got your NCERT books. That's more than enough for the most of the chemistry, you'll learn.

For any degree course you will need to clear your 12th, so you should focus on the same.

Yes, you can but it also depends on yearly cut-off too.

Some of the top colleges for computer science engineering are mentioned in the link provided below:
https://www.shiksha.com/b-tech/ranking/top-computer-science-engineering-colleges-in-india/46-2-0-0-0

Where do you want to take admission? Please specify.

If you are only worried about scoring marks, no need to study books thoroughly, just download previous 5 -10 years paper and go through it thoroughly. It is sufficient to score 65+ marks.
I have already tried it.

Hello Gunjan,
You have many option like you do diploma courses or B.Sc and also B.Tech if you have interest in engineering other engineering, you can do BBA or B.Com and many more courses.
All the best.

My dear friend,
If you have the qualities required for the job, then you will get the job. Scope doesn't matter in that case. So, find your field of interest and don't think of scope, just go for it.
All the very best.