From my experience, only practice can help. All you can do is look for the hidden image, no real shortcut to it. But with practice you'll get better.

Don't think about this now, just focus on your studies and try to get the best result out of 12th exam and NATA 2019 Exam.

To increase your chances of getting a seat in JJ College of architecture you have to target 150+ in the NATA exam

Definitely you are eligible to apply, Please apply .

There are some great tips here - https://edge.dqlabs.in/articles/how-to-prepare-for-nata-in-1-year

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.

Could you be specific and clear as to what you want to ask cause I didn't got your question??

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