JEE Eligibility

You didn't satisfy the minimum board's criteria to get admission to NITs. Unfortunately, you can't get admission.

You can contact the authority:
https://jeemain.nic.in/WebInfo/Public/View.aspx?page=78

You can apply 2 conductive years after passing class 12th. After that, you cannot apply for JEE.

No, you are not eligible for JEE Mains in 2019. It is stated in the guidelines of JEE Mains the maximum attempts are 3 in consecutive years.

JEE Mains can only be given 3 times. If you have not appeared for JEE this year, then you can apply for it next year.

Since you are from SC category, you can get a seat if you have a score of 100 and above in JEE Mains. I don't think it's good to drop a year.

Yes, you are eligible.You can apply for a seat with this score.

It is difficult to get the CSE in IIT because IIT, the OBC students also want a rank under 500 so for IIT institute, you have to choose the another branch in IIT or search the criteria which may be changed. All the best.

There is no firm and fast degree requirement for security analysts. Nevertheless, most employers are going to be looking for a Bachelor's degree in Computer Science, Cyber Security or a related field. So, you see that through JEE Mains, you get a college and take CS or IT branch which introduces you to the basics of the any IT or computer-related jobs and security analyst as you mentioned.

As you know, every student works hard but toppers have a different strategy of smart study which differentiates them from the crowd. The secret of getting more than 250 marks is focusing on those topics which are not asked or very less often asked in the paper. Secondly, revision of topics and paper solving
Thirdly, giving mocks. Do these things and surely success will be yours.
All the best.