CBSE

Hello,
I hope you are doing well. The JEE main paper 2 marks are valid for NITs and IITS to getting admissions but most of the government and private college do not consider JEE main paper 2 rather the consider NATA score. If you had given NATA then you will be able to get NIRMA University, MS University, CEPT University, etc. They are the top universities for B.Arch.

Of course you can give NEET exam, because it is not for any particular board. Any student from any board can give this exam.

Please check for your eligibility to appear for the IISER Entrance Exam from the criteria mentioned at the link provided below:
https://www.shiksha.com/science/exams/iiser-entrance-exam.

You can get chemical engineering in private colleges with these ranks. Or, you can go for other courses in science field.

Min 45% required for General category student.

No, it is not mandatory to appear for the optional exam. The name suggests it is 'optional'.
You need minimum 5 subjects (including a language). But I'll advise you to appear and write CS exam also. In future, you'll have many fields open for you. It'll prove to be beneficial for you.
Work hard & good luck!

Dear,
You can get admission in it's my hope but there is a chance that you can get admission if they lower the cutoffs.

I guess the admission is based on JEE rank.
Tell me your rank then I can help you.