CBSE 12th

eenough percentage for 99 percentile and 250- 290 JEE main score.

It depends. If you get call for counselings then definitely you have chance for NITs.
All you need to do is to wait for counselings.
All the best.!!!

if you don't want to give another year for preparation of JEE Mains or any exam, then go for engineering, as far placements are concerned, don't worry about that.Give your best in engineering, if you check online, you will find that there are plenty of opportunities after engineering also.At last, never lose hope.

The students who do engineering, are never average, they are beyond and above average, and they don't remain poor as engineering knowledge makes everybody richer from mind.

I suggest to you what I have done to many others. Please do your Bachelors in India alone. You may want to study your Masters abroad. To study a Bachelors in Engineering you have to spend at least INR 20 lakh rupees on living and tuition per annum for four years. Unless you have that kind of funding ready, it is unwise to study a 4 year Bachelors degree abroad. Hope this helps. Thanks.

The cutoff for class 12th marks is 50%, but there is a continuous scale and you will be almost at the bottom of the list. Hence to make up for that, you will need to score more than 99.7 percentile in CAT.

CS is always better than any course but if you have deep interest in mechanical then take mechanical

There are many good and prestigious colleges which accept JEE Mains score like DTU,Delhi . Here is a list of some colleges .Check them out-
https://edudrona.com/questions/question/what-are-the-top-colleges-accepting-mains-score/?mref=tanu

It depends on what you want to do in life after pursuing the course.