Exam

You must get above 85%ile to get a good college.

Yes, you are, but in the improvement, you must have scored 75% only then it will be useful other than that it is just a waste of time, energy and money.

The only eligibility criteria for admission is that you must have science in class 12th from any recognised board and must have secured 75% in the board exams.

No, this can't be possible as only for admission in IIT you need to qualify the Advanced paper and only then you will be able to get admission into the IITs and through the Mains score.

Yes, you can try your best. I'm providing you with a link, you can visit this link and then you can read all the terms and conditions of the scheme.
www.jeemain.ac.in

No, both the papers have different rank and different score and according to that, you will be able to get admission in the colleges only, the counselling is same and all the other things are totally different for both the papers.

You can get almost all the good NITs some in core branches and some in non-core branches and also in some low ranked IITs.

For JEE Mains, you can appear for 3 consecutive years and for Advanced, you can apply for 2 consecutive years only.

Yes, you can easily get some top NITs and IIITs but I'll prefer you opt for IITs as the name tag of IIT is better than a good NIT also every IIT pack of every branch is good.