Polytechnic

If you go with BCA, top MNC's do not prefer BCA students and even it might take a longer time to settle in the career. But, if you go through Computer Engineering (Polytechnic) you will have both dual degree as well as sounding knowledge. So, computer engineering is suggestible from my side.

If you are done with the diploma , Then you will get admission directly in 2nd year B.tech . It is mecessary to complete diploma in the same branch.
If you are fulfilling the criteria then you can get admission.

Yes, for a polytechnic course, you need to get admission in a college, but for a computer course i.e, basics, C, C+, etc., you can join a computer centre or an institute.

No, it's better to take admission in B.Tech, your one year will be saved.

Most banking exams require 12th standard. Less probability of applying through Polytechnic.

Yes, you can go for Diploma in aeronautical engineering after class 10th, and after the Diploma, you can go for aeronautical engineering, it will be easy for you.

After completing your Diploma, you can apply for a lateral entry B.Tech admission. You continue your direct second-year graduation. All the universities of our country offer this type of entry.
All the best.

The subjects for the 1st semester are:
Chemistry
Engineer mechanics
Language electives
Mathematics
Physics

If you want to do engineering then my suggestion would be do polytechnic and then degree as you get a good knowledge of your field as in polytechnic there is much practical oriented study.