Degree

Firstly, congratulations on your rank. And you can easily get into Sree Vidyanikethan Engineering College in ECE department. The fee structure is around 2.2 Lacs with a hostel as per my knowledge. So, contact the admissions desk of the college for further information.

A degree in aeronautics will only help you in understanding the concepts of aerodynamics. Although, it will not add to any specific or known advantage, while pursuing pilot training or interview for that matter. The aviation regulatory authority has decided minimum qualification for a pilot to be class 12th from any recognized board in PCM. If you have not started your B.Tech. then my personal advice is to not to go for it, because it will take 4 years to complete the degree, in which you can easily complete CPL and get a pilot's job.

Graduation is must for MBA. No matter what the stream is. But, to pursue MBA in maritime, I would suggest doing B.Tech in Maritime too. It will be very helpful in your career.

Hi,
PG admissions are done. Lectures have already started. You can try next year.
All the best.

Hi,
IGNOU degrees are recognized world-wide, however, universities have their own admission protocols. Singapore B-schools may require an additional verification which you will know if you enquire with them directly.

Here is a list of some popular colleges in Pune for Engineering:
https://www.shiksha.com/engineering/colleges/colleges-pune

This is a nice question because now everyone is busy in jobs or some students have some personal issues from which they can't able to attend regular courses but now, there are top distance education universities in Mumbai, Pune, Delhi, Bangalore and all over India from where you can get same degree as you get from regular colleges. Moreover, there are two possibilities of complete graduation in one year which don't affect n your degree value.

Hi Shyamal,
Your question is bit tricky to understand. I will try to solve at my level. If you are saying that is it possible to get admission in CS after completing diploma, the answer is yes, you will get admission in direct second year of engineering in CS branch. Also if you wish to change your branch in engineering then you can change it.
Hope this helps you.
Best of luck.

https://www.shiksha.com/aviation/course/aircraft-maintenance-engineering-in-avionics-and-mechanical-stream-hal-pravara-aviation-institute-nashik-132168
It is a UG course, as stated in the link.