Chemistry Spl

Yes, you are eligible.

Yes, you are but choose a relevant subject in last year of your graduation. Go for botany and chemistry if you want to do Masters in chemistry, botany or microbiology.

If you have studied botany in the last year of your graduation then for sure you can apply for M.Sc botany.

No, you must have done UG in agriculture.

You can pursue PGDCA, PGD in library science or Diploma in journalism.

No, you need to have a B.Sc degree in agriculture.

If you're interested in some sort of career, plan it accordingly. For example, you want to be a chemist, go for a B.Sc in chemistry and similarly, to be a physicist or if some research drags your interest in physics, then, be on physics!
If you are opting to settle for better, then, B.Sc chemistry is for you. Physics degree must be accompanied by a Master's degree, but a chemistry Bachelor's degree could do it better in fetching you a job! I, personally, had my uncle go for a job in a pharmaceutical company after he completed his B.Sc PCM (physics, chemistry and maths).