Science

Hi Pradyumn,
Yes, you can pursue astrophysics but what about the medicine knowledge you have collected in MBBS. It's all about your passion, I know a lot of doctors who went into other fields like civil services, management, research, acting and so on. If astrophysics is your passion, go for it.

I am suggesting you to wait for the cut-off list, 2019, and then you'll get to know the exact chances of your admission. Though you have a good percentage in OBC category, may be you'll get a good college (but not the North Campus).

Yes, Dyal Singh College has B.Sc. Hons in Botany course. As Dyal Singh college comes under University of Delhi, so take admission in this college. You have to first fill the registration form for Delhi university (DU) and then wait for the release of cut offs and then you can apply for the admission in Dyal Singh college on basis of your 12th score only if you met their cut-off requirement. For more details about Dyal Singh College, refer to the following link:
https://www.shiksha.com/college/dsc-dyal-singh-college-lodi-colony-Delhi-2319.

I suggest you to go with B.Tech CSE which has good scope and better opportunities.

Check the below mentioned link for -
Details: www.paruluniversity.ac.in
Apply online: www.puadmission.in

No, it is not necessary while pursuing B.Sc (hons-maths) to have Physics.
You can pursue B.Sc (hons-maths) even without studying Physics.