Science

The one and most important benefit for taking Science after 10th is you can go in any field after 12th rather its arts or commerce but you can't go for Science courses with arts or commerce subjects. But, you should not consider this fact and go solely on your area of interest.
Both courses are important on their own part.

Not possible, maybe you can contact the institute, but I am not sure.

Both, M.Sc. and ME have research based and course based program. ME will contain more hardware papers whereas M.Sc. will concentrate more on SW. After M.Sc., you can go for PhD. Length of both the courses is 2 yrs.

Below mentioned are the best universities in Canada:
1. Ecole Polytechnique.
2. University of Calgary.
3. McGill University.

You must go for USA.

You can do B.Sc. and or BA, they are good.

MCA is a job oriented PG degree with more job opportunities in comparison to M.Sc degree. But if you are of scientific temperament then M.Sc is a better option. You may chose any based on your interests.

Jobs are available at qc/material testing laboratories. Masters will lead you to government jobs.

Certain Universities permit both combinations, though a mathematical background is very much advantageous for Chemistry and Research.