Ant's eye view will no way determine the stability of the object. If your rough sketch in 2D is stable, then the 3D one would be too. According to me you needn't worry about whether it looks stable. You may not realise it, but it will look stable :)
Past year question paper is available on DQ Edge as Tests segregated by topic. So, you can practice and get the solutions + know where you stand again'st others taking a test which is much more effective while preparing for NID or any other entrance exam than getting the solutions directly.
1-You need to practice for NIFT CAT exams from sample papers which are available online.
2-To improve your drawing skills- Practice, practice, practice.
Your have scored very less in 12th so it is very difficult to get a seat in J.J College of Architecture. I will suggest you to look for other colleges
No, physics and chemistry is not asked in the NATA paper.
For your reference, you can go to the learn section to know which topics you have to prepare for the NATA Exam.
It will better if you write an email to DTE Maharashtra citing the technical glitches you are facing while filling the form. They are in a better position to guide you properly.