Polytechnic

Yes, you can study computer science Diploma in government Polytechnic College with 85.40% in class 10th board exam. The minimum eligibility criteria to apply is that a candidate must have passed class 10th examination with a minimum of 35% with mathematics and science.

1. Bengal College of Engineering and Technology
2. Jalpaiguri Polytechnic Institute.

Here is the list of good colleges for Diploma in civil engineering:
https://www.shiksha.com/engineering/colleges/diploma-civil-engineering-colleges-west-bengal

Yes, you can definitely think of doing B.Tech after the completion of your Diploma.

If you have done polytechnic then you can get admission in direct second year otherwise, you have to start from the first year itself.

Yes, you can do BBA in airport management after doing Diploma in ECE because for BBA what is needed is 12th or any equivalent without any specific subject. The course of airport management has scope in aviation and flying. However, before that you should be a graduate in a particular stream from a recognised college or certified institute. Other BBA specializations like oil and gas management, and financial marketing also have good scope.

The semester wise detailed fee structure for courses of University Polytechnic, 2018 batch, can be viewed here:
https://www.bitmesra.ac.in/Show_Content_Section?cid=12&pid=351.

Yes, it will be right, no problem. However, it is best when you do it after 10th.