JEE Main

No, in Tamil Nadu, the best college is NIT Trichy and Anna University. For both it is difficult as for NIT, the minimum marks are 75% in HSC and around 4000 all India rank for B.Arch. I would suggest a private college in Bangalore or Hyderabad.

Yes, if you are an authorized citizen of India, or NRI citizenship, enrolled in any higher secondary academic programme from a recognized board, and not born before 1996, then you are eligible for it.

There are very few seats for general in both the colleges. Also, in NIT, minimum board percentage should be 75%, so you will not get NIT.
For Sir JJ, the admissions are through 50% board + 50% your competitive scores. Your board percentage seems less, so the chances are very thin.

There is every year cutoff, depending upon students and total no. of college seats available. Though, you can expect a cutoff around 85 i.e. minimum marks to get through JEE Mains.

Mere clearing the JEE Mains will not lend up any college across the nation, atleast a good one. Kindly mention your precise marks to get the best suggestion. Moreover, you can use the college predictor too for searching the colleges.
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor

Though, you cannot get into any of the NITs. You can get into some autonomous institute with a bit lower rankings. Also, with home state quota, you are expected to get a college with a relatively decent branch. Kindly refer:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor

Yes, you can get into GSITS Indore with spot round admission. It is in top 100 in NIRF rankings. You can get into RGPV University (Bhopal). For more possibilities, I would suggest you going through some state universities and autonomous institutions as well. Also, if you have marks more than 100 then you can be into LMNIIT, Jaipur also, Rest, private and other government. Universities are there for which you need to clear their entrance.

You might not get as there are very less government colleges for architecture. You can get into good private universities for pursuing B.Arch. I would suggest you to try:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor
To get a brief of what you can expect with these marks, Manipal and Jaypee College are some recommendations.

Hi,
Kindly google in the previous year opening and closing rank of NITs to get a rough idea of which colleges and which branches you can apply for. As for IIT they refer to the JEE Advanced marks not of JEE Mains.

No, your score should be more than 75% in 12th and around 200 in JEE Mains paper 2 for private colleges.