I am not sure I understand what your query is. If you get very low marks in JEE Main, you can write the exam but admissions will be a problem as you should get either 75% in your class 12 or be in the top 20 percentile.

See if you give NIOS and provide the other certificates from your CBSE attempt, that wont work. Why you wish to write NIOS is something that I dont understand. You could go for improvement exam in CBSE.

Please explain what you want to know so we can help out better.