Exam

No, bro. You can't NIT admission is based on JEE Mains. But, you can get other colleges on basis of 12th Marks. Use Shiksha's College Predictor for checking your chances, here for getting any College on basis of JEE Mains:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor?utm_sourc

B.Tech programme for all the colleges and universities is 4 years. No question of Doubt.

No, you can't get admission in the colleges which only accept JEE Mains marks. You can get colleges of your state, maybe even government ones. Try JEE college predictor to get more information and insight about college which you can get on the basis of your rank-
https://www.shiksha.com/b-tech/resour

You can apply for the university who takes their own exam for the admission.
Examples includes VITEEE, SRM, BITS, AMUEEE , KITEEE, WBJEE, etc.
Best of luck

Check your Chances here at Shiksha's JEE Mains college Predictor. Link is given below:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor?utm_source=connect&utm_medium=CR_JEECP&utm_campaign=6540165.

Check your chances here at Shiksha's JEE Mains college Predictor. Link is given below:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor?utm_source=connect&utm_medium=CR_JEECP&utm_campaign=6540165.

Check out this link for your chances of getting a college with JEE Mains Ranking:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor?utm_source=connect&utm_medium=CR_JEECP&utm_campaign=6540165.

You will try two times for JEE Mains.

You can start preparing from today itself. If you have ample amount of time join a coaching class near your residence or else go for online course. There are many tutors like for eg. Ravindra Babu. His videos are really helpful.

https://www.shiksha.com/mba/course/master-of-business-administration-kvm-college-of-engineering-and-it-kerala-other-213036
Kindly visit the link. I hope it helps.