B.Sc

Yes, you are eligible for both the courses. But, you will have to appear for entrance tests and get a good rank to get admission in the top colleges.

You can opt for M.Sc or MBA.

B.Sc in Aviation and Airport Management is related to all about management of aviation industry. This is a good course but as of now, very less scope is available. If you want a good career in this, you can pursue MBA in Aviation Management after your B.Sc. and also in India many Airports are on verge of Privatisation. So, it will give you good options in future and also, many airlines would be coming in future. But time period for this growth is around 10 years.

B.Sc. in chemistry is 100 times better than B.Sc. biotechnology, although biotech has a great market abroad but in our country there is no particular job for this. You can go through biotech in M.Sc. if you want to be a biotechnologist, but I would suggest you not to take biotech.

You can go for MBA in tourism if you are passionate about travelling.

Some of the best options include:
Armed Forces Medical College (AFMC)
Centre for Hospital Human Resource Development
Tehmi Grant Institute Of Nursing Education
Symbiosis International University's SET- Symbiosis Entrance Test (Undergraduate Degree Programmes)

Bombay Flying Club is the best flying club for CPL and aviation in India.

Yes, you can apply for B.Sc. But remember that both are different fields and you have to start from the beginning for this.

Yes, you can take this branch if you had PCM in 12th.

The B.Sc (CS) course mainly focuses on the mathematical and theoretical foundations of computing, rather than teaching specific technologies that may quickly become outdated. This course prepares you for higher education.
BCA is more professionally oriented; it prepares you for post-graduate or research work and also creates professional manpower for problem solving through application of computer science at the interface of business and technology.
If you want to join Indian Army after BCA, you should give CDS entrance exam as it is for officers post in army, you can join Army after clearing CDS.