B.Arch Eligibility

Anyone who has cracked Paper 2 is eligible for counselling.
However, you may not get your desired college as for decent colleges, the minimum board percentage required is 75%, It is 65% for OBC, ST and SC candidates.
Kindly keep that in mind.

Yes, CBSE also conducts a separate examination for B.Arch.
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Yes, you will be able to apply to IPU. But don't get stick to IPU. Only apply for other universities also where you can get better colleges than IPU.

Yes, you are eligible as you have mathematics as additional subject.

Qualification
NATA
Age
Maximum 21 years
Other eligibility criteria
Candidate must have passed 10+2 with Mathematics as a subject
OR
10+3 Diploma (any stream) recognized by Central/ State Governments
OR
International Baccalaureate Diplomas, after 10 years of schooling, and with Mathematics as compulsory subject of examination

I don't think that B. Arch is there in MNNIT especially at undergraduate level. I can't say clearly about post-graduation. But I strongly think that you have confused between MNIT to MNNIT. So please review it.

Eligibility for the Bachelor of Architecture (B.Arch):
The duration of the course is 5 years and to get admission into B.Arch. The student must be fulfilled the following conditions:
10+2 or equivalent with a minimum of 50% marks along with the compulsory subject of mathematics.
10+3 years Diploma in any stream from a recognized university/ board/ institution.
Cleared NATA (National Aptitude Test in Architecture) with the 80% marks conducted by Council of Architecture.

Here are the subjects of B.Arch:
1. Architecture design
2. Building construction
3. Theory of design
4. History of Architecture
5. Building science
6. Building management
7. Architectural drawing
8. Computer and software lab
9. Workshop practice
10. Architectural appreciation
11. Theory of settlement