JEE Main

Officially you are not eligible to give JEE Advanced if you change your category. Please do not change it as at the time of admission all the details are checked thoroughly and you will be at a mess if they recognise you have changed the category anywhere. So, keep it as it is and if you're not satisfied with this year's rank then drop a year and give the examination once again.
All the best.

Yes, you can get a decent enough state government college depending on your state with percentile, but I don't think you will get into any NIT, especially for core branches.

As this year's cut-off is based on the percentile rather than the marks out of complete marks i.e. 75 out of 360. That's why the cut-off seems so high. There is even a chance that anyone could have got 70 out of 360 and has a percentile of 85. So, by just looking at the percentile you can't decide or say what was the minimum marks to be deemed qualified.

You can get admission in good private colleges of Tamilnadu like Amrita School of Engineering, SRM, PSG, SASTRA University, TCE, Madurai, etc. You can select the branches as per your interests. You can also predict your college here:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor
Best of luck.

Kindly visit this link:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor

No, you can't get NIT with this percentile in JEE Mains.

Yes, sure your admission is confirmed in NIT because you belong to the SC category.

Yes, you can apply for IIIT-Delhi. Your marks are too good.

Your JEE Mains Paper 2 score is very good. You will get top NITs for B.Arch. You can predict the possible colleges here:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor
Best of luck.