Colleges in Delhi

You need to score more than 150 in order to get admission in any of the IITs.

Yes, you can get admission in Jamia. Just apply with your Mains Paper 2 for admission in B.Arch.

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.

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.

It depends on the college in which you are taking admission in.

Shiksha college predictor will help you:
https://shiksha.com/b-tech/resources/jee-mains-college-predictor
Good luck.

If you are a drop out, your 5% marks will be deducted.

Yes, you can make changes in your application form in DU but only till the last day of registration, after that you will not be able to make any changes in the form. The last day to make any changes in your application form for UG courses is 14th June 2019.