Which engineering branch should I opt for in B.Tech as I have a keen interest in chemistry and maths?

See, Application of maths is in every branch of Engineering , but if you having very much interest in chemistry then B.Tech in Chemical Engineering would be best field for you in the engineering field. You can have a good scope in this field if have much curiosity in this field. Best of luck and wo

Yes, you can. But mechanical is good enough for mathematics and chemistry. But if you have very much interest in automotive, you can chose it as your passion.

You could always consider chemical engineering. It has immense scope abroad and offers good pay as well.