Hey Santosh,

You didn't complete your question but yes you can write JEE when you are in an engineering college and if you get an IIt or NIT, you can take admissions but you have to check with the office of the college you are in because they may ask for money and the fees you've payed will not be given.

Yes, you may eligible but after crack the exam. then somehow you pass the exam then you again start your B.tech from start and I've not suggested you that option.

All the best

Even though you take admission into any College and start studying it would not make any difference as you would still be elligible to participate in the counselling session or if even want to reappear for the jee main you can still go for it and attend the counselling session.