Science

If you have already done 11th with science you cannot move to 12th for humanities however you can opt humanities in BA. If you opt humanities in Bachelor in Delhi university, there will be additional 2.5 percent penalty like if cut off for a 12th humanities is 92 percent, you have to score 95.5 percent however in Mumbai University 30% students have quota to opt from.other than the same faculties.

Yes. In most of the institute, 3 years state polytechnic diploma is considered equivalent to 10+2 and if it is IT computer science diploma then you are also eligible for direct 2nd year admission in B.Sc IT and computer science as of now Mumbai University, Madras, Madurai Kamraj, Annamalai offer this facility.

There is no point on doing B.Tech after completing B.Sc computer science and recruiter may not take this positively. Better, you should decide to do B.Tech now only as B.Tech from a good institute has more job prospects or after B.Sc computer science you should opt MCA.

Sorry, it does not offer B.Sc in nursing course. You can refer to the college link for better understanding:
http://www.nuv.ac.in/

Try your luck by submitting application in each and every college available in your state. Though college is not good but you can maintain your skills and goodness to excel in your field.

For an average student the best course to pursue is graduation in any stream and appear for UPSC or SSC exam.