JEE

According to me, you may get a seat in NIT.
But for a clearer idea, you should visit josaa.nic.in
and check previous years' cut-offs of NITs of home state and other states quota and also for various categories (general, OBC, SC, ST, etc).

Civil Engineer is one of the branches of engineering which has a very good scope of government jobs. Government departments like IPH, PWD, Electricity requires civil engineers and many posts are available in every state of our country. Apart from there, central government jobs are also there on the basis of IES exam in CPWD. So civil engineering is very good option for the aspirations who want to pursue government jobs.

No chance of getting NIT. But, there are also other good colleges
Try in them.
All the best :)

No, only NCERT books are not sufficient. You need to study much and more book than that. Some of standard book you need to study and solve them. More the practice, more you will be able to answer in the exam. Search for best books for JEE Advanced study.
Best of luck for future.

Your rank is good and you can get NITs in this rank. Go for the link below to find the Colleges of your choice and Branch:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor#.

There are very low chances of getting any government college but you should try for other colleges. And, you should also go for the state level counselling. Otherwise, you can go for the private colleges across the India. You can get a better one.

You need to have 45% marks in PCM in Class XII.

Hello,
Hope you are doing well. I suggest you not to drop for an year because you have less marks in JEE and you will again start form scratch to get an IIT. Drop will waste your 1 year and the competition will be higher in upcoming years. Go for some good private college with your favourite branch and do well there. All the best.

For going abroad, your NATA exam will have no value and for going abroad, you have to give some language proficiency tests like TOEFL or IELTS. Then, you need to apply separately for different colleges and universities. And also, you have to arrange for VISA.

Yes, it is compulsory to attend the 1st round because it gives a choice and options to choose other institutes for the 2nd round.