NIT

Your chances are too low because you need at least 75% in 12th to get admission in NITs.

Yes, obviously, you have to score 75% in boards to be eligible to get admission in any NITs.

IIT Roorkee offers admission on the basis of AAT (Architecture Aptitude Test).
AAT is an architecture entrance exam for admission into IITs. Moreover, for appearing in AAT you have to clear JEE Advanced also. To appear in JEE Advanced you should have cleared JEE Mains paper 1.

There are chances for you to get NIT as you are from reserved category but your state should have NIT with architecture branch.
Hope it helps.
All the very best.

Can you tell your mains paper-2 rank and your category since fir B.Arch, you have to fight paper.

I dont think all NIT's have this provision but yes there is a provision of branch change after 1st year though the requisites for the same may differ
Like in NIT Kurukshetra you have this provision. You can apply for branch change provided you dont have any backlogs in the first year and leaving your current branch doesnt let the strength fall below 75% of the total strength(including the vacancies) present in your current branch. And cgpa is not an issue if you fulfill the aforesaid conditions.

NIT-Trichy, NIT-Warangal, NIT-Calicut, MNIT-Allahabad, NIT-Durgapur.

NIT-Kurukshetra is the best option for CSE/IT.