JEE Main

I think peter sykes-reaction mechanism is the best book for organic chemistry.

It is very difficult to get NIT/IIT. So, you can try for IIIT, other state government college, private college. Some private colleges are Thapar University, Chandigarh University, BITs, Manipal University, VIT etc.

Sorry. According to me you don't get any nits or iits.
It will be tough to get good government colleges but you can try for some good private colleges. I am attaching a link which will predict the best college according to your rank.
Try the college predictor:
https://www.shiksha.com/jee-mains-college-predictor

The last year the highest closing rank in NIT-H was 235452 in your category in civil engineering in the special round under HS quota. Your expected rank is about 241381. Your chances are moderate. However, JEE Main April can provide you with another chance for improvement.

The candidates who have passed XII examination in 2017, 2018 or appearing in 2019 irrespective of their age can appear in JEE Main examination 2019. The candidates should satisfy themselves about the eligibility of age limit requirements of the Institutes in which he/she is desirous of taking admission.

You will get the admission in NIT, but the branch may or may not be of your preference. It all depends on how you choose the college and the branch.

You can check the probability of getting a college here:
https://www.shiksha.com/b-tech/resources/jee-main-rank-predictor

If you want to get a good college and you have enough dedication and confidence. You should drop one more year rather than doing B.Tech and preparation simultaneously. B.Tech has its own wide syllabus, it will not going to be easy to accomplish both task together. If your college is good, then focus on your academic and learn skills to get future ready. If it is not you can dream and dare to get an IIT for higher studies. It will be a wise decision.

Yes, you can but your board exam's score will affect a lot.