CBSE

You are not likely to get a government college in India. Hence, you'll need to get either into big private universities or into local engineering institutes. Check the best colleges in PUNE for Engineering or either apply for MANIPAL, VIT, AMITY, AMRITA or LPU for distance education. NIMMS also if you can afford.

If you have any quota, then maybe you can get admission.
It also depends on this year's cut-off.

Firstly, you can go through the chapters which you thought you can easily deal with it and do more questions of it.
Then step by step, you can go to slightly tougher chapters.
It will increase your confidence.

No, because in science stream, PCM are the main subject and also you can not do chemistry or physics honours. Because they also require maths. You may go for hons in physical education or hons in English or other humanities stream subject courses.

Yes you can do B.Sc. with 69.99%.
You are eligible.

Top Engineering Colleges in India are mentioned at the link provided below:
The ranking is based on rankings published by India Today, The Week, and National Institute Ranking Framework (MHRD). https://www.shiksha.com/b-tech/ranking/top-engineering-colleges-in-india/44-2-0-0-0.

Top Computer Science Engineering Colleges in Delhi/NCR are mentioned at the link provided below:
The ranking is based on engineering ranking published by The Week. https://www.shiksha.com/b-tech/ranking/top-computer-science-engineering-colleges-in-delhi/ncr/46-2-0-10223-0.