B.Sc Physics

Yes, physics has good opportunities like in the fields of nuclear Physics, Nuclear Medicine & more.
All the best.

M.Sc. (Information Technology): Eligibility
Candidates can be eligible for admission if they fulfill any of the following conditions:
Bachelors degree in Information Technology / Computer Science / Physics / Stats / Electronics or equivalent examination with at least 45% marks in aggregate from a recognized university.
B.Tech./BE degree holders in Computer Science/Informational Technology are also eligible for M.Sc. IT course.
Aggregate percentage required in graduation or equivalent may vary from college to college.

You may or you may not. Every college has its different guidelines, so contact to your desired college.

Yes, you can complete your B.Sc, but you have to submit gap certificate with a valid reason.

Please refer to the link provided below for details regarding admissions in FC, Pune for a course in B.Sc Physics. You can download their brochure which contains all details regarding admission criteria, course eligibility, fee structures etc. Considering you have scored 84% in your exams, it should be an added advantage for your candidature.
https://www.shiksha.com/college/fergusson-college-fc-pune-fergusson-college-road-25748

You can prepare for IIT JEE and other engineering exams simultaneously. If you get selected next year and get admission in any reputed institute then you can drop your B.Sc and go for B.Tech. Otherwise other option is to complete B.Sc and then M.Sc. After that you can pursue M.Tech.