Architecture

No, that is not possible. BE / B.Tech( Mechanical Engineering, Civil Engineering, Marine Engineering, Naval Architecture) is a Mandatory Course.
Check out the details below:
https://www.shiksha.com/engineering/course/m-tech-in-naval-architecture-and-ocean-engineering-indian-maritime-university-chennai-visakhapatnam-campus-276778.

Mathematics - 20%
Aptitude - 40%
Drawing - 40%
So, it's better to concentrate on Aptitude and Mathematics if you are good at drawing.

The final AIR will be declared after April 2019 considering the best score out of the two JEE Main attempts for all the applicants, hence based on current percentile we cannot predict whether you will qualify for NITs/SPA or not. Wait for the final AIR declaration
Details:
https://www.shiksha.com/b-tech/articles/how-to-calculate-jee-main-rank-from-percentile-blogId-19953.

The final AIR will be declared after April 2019 considering the best score out of the two JEE Main attempts for all the applicants, hence based on current percentile we cannot predict whether you will qualify for JJ college or not. Wait for the final AIR declaration
Details:
https://www.shiksha.com/b-tech/articles/how-to-calculate-jee-main-rank-from-percentile-blogId-19953.

First of all, congrats for you. You got a good score & can secure seat in Sir JJ College of Architecture.

There are less chances to go into this college. As there are already a few seats for architecture and this institute has the cut-off rank of 1721 for outside state students.
You may check Shiksha College Predictor to get more idea of which college you can get.
You can opt for any other college. Hope you do well in your future. Wish you all the very best.

No, you have to appear in the JEE Mains paper 2, then only you will be eligible for B.Arch.

Hello,
Both the exam tests have similar topics and difficulty as well. It depends, if your idea is clear and focused towards CEPT only. Then, taking CEPT entrance is an option. GATE-AR would open more colleges for you.

Actually, NTA considers your candidature based on the 2 exams in which you appeared. So, be prepared for that.