Government & Defence Entrance Exam

Yes. Students of a category compete among themselves. They are selected on the basis of their category rank.

Are you talking about Amity University?
http://www.amity.edu/Bihar/amity-Patna.aspx
If it is what is in the link, then it is not fake. You will be eligible for all graduate based exams.

NCC would be helpful for Police. NSS is just for name sake. Also, Income Tax recruitment is done mostly through SSC Exam.

There are some vacancies that are solely released for O level certificate holders. Keep an eye out for them. Also, some jobs give preference to O level holders like UPP, UPSRTC, etc.

Managing both will be hard. Since you are pursuing Honors, you will require proper time management. You can start by studying newspaper daily and Studying General Science. Read the Newspaper thoroughly in morning and Study GS in evening. When you get holidays, pickup a topic from history or civics and finish it on the same day.

The syllabus of UPSI and SSC CGL is almost 70% same. Keep doing SSC CGL coaching to prepare for UPSI. It won't make a big difference. However, UPSI has two more topics that are not covered in SSC coaching, Hindi and UP GK. For these two, you can join another coaching for one hour.

If you are getting a good college for MBA then study MBA. If not, it would be wise to prepare for Bank and SSC. And since you have already done M.Com. It is not worth pursuing MBA if you don't get a good college.

You can clear SSB without actually having to mention your eye vision. SSB doesn't test eye vision. Medical tests are there for that. You will not be able to join the Air Force. You may be able to join the Indian Army if you can clear their the lowest acceptability.

If fluency in Kannada is a requirement then the inability to write in the Kannada language may be considered as an ineligibility.

If you are looking for direct entry then probably no. Since you failed in one subject it would take time to clear that and you can become overaged. Or you may not be able to get enough marks to qualified for SSB. The percentage required for direct entry are quite high and are always above 90%. You can however prepare for NDA later.