Bachelors

Yes, Australia a hub for mining engineering and job opportunities related to it. It is also good for mechanical. The three branches are different so decide what you want to study, pick just one. For CSE, Canada and US would be better.

Possibly, Political Science, History, Economics, or Geography. Choose whichever out of these, you find the most comfortable in as these are the most helpful subjects to ace UPSC.

Yes, you are eligible to get admission in B.Sc after completing class 12th.

You won't find any in SPP colleges. You will find certificate programs like mechanical engineering technology. Bachelor's degree are also offered but not in aerospace engineering. You will find the same only in the universities - Carleton University, Ryerson University and University of Manitoba to name a few.

If you study in USA you will be eligible for practical training visa in the field of your specialisation. You can work maximum for 18 months under this scheme. Your employer will have to file your H1 visa. If you are lucky in H1 visa lottery then you can work for 5 years more in USA. You may visit USA embassy website for further details.

There is huge demand and supply of CSE professionals in USA. Most of the demand is filled by Indian IT companies by placing IT professionals in USA. You will definitely will able to get job in USA after CSE graduation in USA universities. But you may face visa challenges to work in USA. Getting H1 visa to work in USA is becoming very difficult nowadays.