B.Sc Maths

Yes, you can join navy by taking Combined Defence Service, this itself provides all the training of arms and ammunition to the cadets. Enroll yourself in CDS and work hard.

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.

for this you have to go to the respective website and apply for the college and also look out for the cutoff

The eligibility for the course is as below.
The normal eligibility criteria for B.Sc Math Honours degree course is a pass degree in 10+2 or equivalent examination in Science stream with atleast50% marks obtained by candidates in Mathematics subject or in aggregate from a recognized board or Institute.

Yes, of course you will get B.Sc Maths in St. Xavier's College.

Check out the link below:
https://www.shiksha.com/science/cucet-exam-question-papers