JEE Main Eligibility

You need to give the NATA exam (preferably before the end of May) and then in the month of June-July apply for the admission process of DTE. If your NATA score and 12th percentage are really good then you might get into Sir J.J. college of Architecturing.
Admission to B.Arch. is done on the basis of valid score obtained in NATA/ JEE Main 2018.

Yes, you are eligible for JEE Mains provided you have 75% in class 12th. It doesn't matter that you have appeared for any other examination or not.

Yes, you are eligible for both JEE Mains and Advance.

Surely you are eligible to give the exam. But you will be disqualified in the document verification round at the time when you report to the college. Hence you should not give JEE Mains.

The only criteria is that you have to fill the form for counseling as per the notification by JOSAA and pay the required fee. Yes, OBC candidates get benefit of their category. Hence there are two ranks - general rank and category rank.

No, first he have to pass class 12th with 75% marks to get admission in Nits, because counselling of josaa is starting from 15th June. And, he don't have that much time.

Yes, of course, you are eligible and can apply for the counselling. So, don't miss this chance and do apply for the counselling. Atleast apply for your "homestate" NIT.
Best of luck.

Hello dear,
Your home state will be the state where you belongs to information of home state is given is your permanent residential certificate. Or voter ID. It doesn't matter where you completed your school study.

Hello, hope you are doing well.
Yes, you are eligible. You have chances to get NITs for Architecture.
You can try for NIT Bhopal, NIT Warangal where you can get admission easily. Otherwise, you can go for top government colleges where admission will be easy for you.
All the best.

Yes, every student who has passed 12th and cleared JEE Mains with the least cut-off marks for their category is eligible for JoSAA counselling.