NIT

No, you won't get any NIT, but you can get any government college in your home state. Be aware at the time of counselling.

It depends on the cut-off for that particular year. But 31 is a very low score to qualify for NIT.

Yes, you can get admission in NITs.

I don't know about the reservations, but for admission in a good NIT, you don't have to wait for the JEE Advanced score, you can get admission to the college by getting at least 250 marks in the JEE Mains exam.

No, you can't as you know you have failed in chemistry. You'll not get admission in any NIT or any B.Tech college.

180+ will be required for NIT Bhopal.

200+ marks will BE require for JEE main to get admission in NITs.

Rank near to 1500 in general-PWD category can get you a seat in NITs for OBC and SC/ST PWD category marks required are even slightly lesser.
All the best.

Almost, all the colleges including the private one's consider JEE Mains marks for admission. The cut-off generally varies from college to college and also on the basis of that year's JEE cut-off.
Some colleges also offer direct admission on the basis of class 12th marks.

Eligibility criteria:
IIT JAM 2018 Subject-wise cut-off is mentioned below. Please check, if you made the cut from the marks mentioned below. In Physics, qualifying marks for general category are 19.51 while for SC/ ST/ PWD, it is 9.76. For Physics Paper/
In Chemistry, qualifying marks for General Category students are 14.79 and 7.39 for SC/ST/PWD. For Chemistry Paper of IIT JAM 2018. In Mathematics, qualifying marks for General Category are 23.35 and 11.68 for SC/ST/PWD. Based on IIT JAM Result 2018, candidates will be admitted to institutes if they fall in above eligibility then you are eligible.
All the best.