NATA

In order to get admission in top colleges, try to score more than 120 in NATA entrance exam. So, you can try in a good college, because you might get top college.

I don't know about IIEST Shibpur as it will obviously factor in your JEE Mains AIR (paper 2 if yo're going for B.Arch and to secure a good AIR with 57% in chose is not possible. (I think they set the minimum cutoff at 60% to appear for counselling). Even though to GET Architecture in JU you need to merely give the NATA exam (the score is irrelevant), even JU has a minimum cutoff of 60% in boards. I'd suggest, if your board permits it, appear for an improvement examination.

Based on the ranking you will be allotted a seat in the college. If you want particular college then reach out to college and take the seat. All the Best.

You have less chances of getting admission in top colleges for B.Arch based on your score and category. But you can try to get admission in an average college.

Average is 135+
But, the admission is not only based on NATA. There is an average of your class 12th Marks and Nata Score. Your NATA score is divided by 2, and then Boards percentage is added. For Example:-
Boards Percentage-60
Nata score-90/200
Nata Score 2= 902= 45
60+45=105/200
Your Merit scores comes out of200.

Yes, you may get a seat in REVA University.