B.Tech

Brother, Diploma in computer science is a bad idea if you have to pursue B.Tech after Diploma. Go directly for B.Tech after HSC, it needs a good knowledge of physics and maths for which HSC is good.

Yes, in all NITs, national level institutes or central funded institutes admissions are done on the basis of JEE mains merit ranking.

I am sorry to say but admission in MNNIT is done through JEE Mains only. Also, the candidate should opt for PCM in class 12th.
All the best.

B.Arch can't be compared to B.Tech because both are professional courses but their ideas and creative mind for thinking is different.
So, be careful about that you can't compare both but you can relate both of them as architecture is extended part of civil engineering.

Yes, in the second round of counselling, you can easily grab a seat in CSE in MIET which would be held later on. So, keep patience and search for institutes that's a best fit.

Your JEE score is impressive, however, you just fall short in your board score by 3%. (Assuming that you are from open category). If you are from reserved category then you will get what you asked for.

Hello,
Please refer to then link to get more insights of colleges and branches:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor