JEE Main

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.

I'll suggest you opt for any branch in a good IIT as there you can get your branch upgraded if you are among the top students of you branch plus the pack of top IITs is awesome whether it is core branch or not.

Yes, you will easily get NIT and IIIT of your choice but the thing is that you should also appear for IITs also as if there is some seats are left vacant then you can easily grab them.

No, you are not eligible to appear for the Advanced paper as you haven't scored 75%.

No, you won't be able to get admission as the most important thing is the eligibility criteria and if you satisfy this then you can even get admission in some NIT but, if not then it will be difficult for you get admission in any central government controlled institute.

The eligibility is:
1) Candidates must clear the JEE Advanced exam.
2) General category candidates must score at least 20% marks in JEE Advanced i.e 73 (varies year to year) marks out of 366 and at least 6 marks out of 122 in each of the three subjects.
3) OBC-NCL candidates must score at least 65 marks out of 366 marks and 5 marks in each of the three subjects.
4) SC, ST and PD candidates must score at 10% marks i.e. 36 marks out of 366.