Architecture Entrance Exam

Yes, you can appear for the NATA exam after completing your HSC.

No, you don't need to give JEE Paper 1, because Sir JJ College of Architecture accepts NATA score & from this year, admission for architecture is also done through JEE Paper 2 marks.

NATA qualifying examination scores are valid for a period of two years starting from the date the individual appeared for the examinations. Admission into an institute accepting NATA scores is not possible before you have appeared for the exam.

For IIT you must clear JEE Mains and Advance and then AAT.

You can appear for NATA but you can't get admission in architecture course as the eligibility criteria is 12th passed with maths and English for science students.
I hope it will help.

You only need a minimum of 55% in class 12th. Qualifying marks in NATA would be governed by the following rules. You must satisfy all three rules to qualify.
1. At least 25% must be secured in MCQ portion (30 out of 120)
2. At least 25% must be secured in drawing portion (20 out of 80)
3. Overall qualifying marks (out of 200) would be based on post-exam statistics and at the discretion of
the council.