NIT Eligibility

Hi,
You can check your rank with shiksha rank predictor and if your rank is below 20k you can get into a good NIT. Click on the link to know more -
https://edudrona.com/questions/question/what-ranks-should-i-get-to-secure-seat-in-a-nit

JEE Mains require more than 75%

It should be more than 250.

this depends on JEE score

As u Are SC, you may get good private colleges with scholarships too. But you should have minimum 50% marks in HSC to match basic eligibility criteria for private engineering colleges of India

Admissions are on the basis of your JEE main's All India Rank. To check wether u are eligible, u can use a JEE main rank predictor and then check opening and closing ranks of recent CSAB counselling at http://csab.nic.in/ChoiceFilling/Result/ORCR_AllRounds.aspx

Yes, you are eligible, but not for JEE Advance.

Sarvesh you can't enter NIT based on Diploma, though you can go for IIT so you have to clear JEE Main first but your score card won't be released , only you will be intimated that whether you have qualified for JEE Advance or not and if you get into IIT then your scorecard will be released