Science

Yes, surely, you will be able to prepare but you will need to learn some elementary mathematics and also focus more on the logical reasoning questions based on basic mathematics.

Without doubt, post-graduation and Microbiology would be better as you will have lesser competition in this filed. Besides, the scope of Mircrobiolgists is increasing fast in India after the implementation of FSSAI clause on all food products. Even the large hotels now employ microbiologists.

Yes, you can change but before that think twice because even with MBA you can apply for the bank jobs. So, understand the requirements and then make the decision. For bank related jobs you need to develop some mathematical ability and logical reasoning power.

M.Sc in Chemistry
M.Sc in Biochemistry
PGDMLT
M.Sc in Biotechnology
Diploma in Analytical Chemistry etc.
I would like to request you to Google for other options too, also take the course you want for your carrier in. Ask yourself about your subject likings and skills.