Class 11

You still have the time and there is no need to worry at all. Consistent efforts and proper time plan will help you perform better in your studies.
Identity your learning style i.e. how you learn better. Is it by doing, by drawing or by reading . Also remember that whatever you study you must practice at least by writing it 5 times.
You can achieve better by being regular and following a routine.

There is no compulsion. You can take the subjects which will help you to score good in class 12th exams i.e. subjects which are easy and help you in scoring maximum marks in PCM group. Also, give more priority to maths as it would help you in your engineering. Do not loose your grip on this subject.

Science stream with physics, chemistry and mathematics are the compulsory subjects.

First of all, you must know the syllabus of JEE Mains and yes you can crack it, but cracking is not enough. Good marks are necessary for getting admission in NITs.

Try to get 80% in 12th if you want to go in reputed university in USA but anyways 75% is also good. After your 12th you need to give GRE and TOEFL exam if you want to go in engineering from abroad.
All the best for your boards exam.

The minimum qualification to appear for CA CPT is 12th from any board i.e. state board, CBSE or ICSE. So, firstly work on basic concepts of commerce while pursuing 12th board.

Maths opens a lot of options for you after completing 12th. So, it would be great if you can take it.

No, you will not face any problem.