Engineering Entrance Exam

You can get any seat definitely in sastra wait upto 3rd counselling after attending 1st counselling. You can get a better branch seat. As you didn't mention any priority of branch.

I think it won't be sufficient to get a CSE seat in SASTRA. If your JEE AIR rank is below 30,000 you can get a CSE seat.

If you are ready to take any branch seat maybe you can get something in 3rd counselling . But you are expecting ECE seat which is not even possible . If you want to apply in stream it requires 970-985 marks in IPE and the Jee AIR rank has to be in below 40,000

The extended last date of online application for BITSAT was 11.05.2020. So you can not apply here with late fee also.

You have less chances of getting seat in your desired college . So opt another college based on your percentile and reservation . Thank you

JEE Main scores are needed for admission in B.Tech courses only. Hence you can do the B.Sc (Computer Science) from Acropolis Institute of Technology and Research. The eligibility criteria and other details are given below.
Eligibility criteria : 12th with Physics-Chemistry-Mathematics/ Physics-Chemistry-Biology
Admission criteria : Admissions to various courses of AIMSR are based on merit positions on the basis of percentage of marks obtained by the student in the qualifying examination (12th).
Total seats : 60
For more details please visit http://acropolis.in/

No, JEE Mains is not the one and only exam to get seat in engineering colleges, there are many exams like EAMCET or the corresponding university entrance exams. JEE Mains is for getting seat in NIT, IIIT basically. But it is very competitive if they qualify JEE Mains only they can write JEE Advanced it is for getting seat in IIT, these are the top most colleges in INDIA so getting seat is difficult. There are many colleges it is based on your locality which provide good education.