B.Sc

Depends on the college you choose, please be specific only then can help you out.
But basic requirements are-
45% in class 12th boards with Science stream.
A good base in PCB.
Preparation for B.Sc nursing entrance exams.

Only and if you have completed your 12th boards with Science. Either PCB / PCMB and have 45% in PCB.

You cannot do both at the same time.

As far as I know, you cannot do both at the same time.

Yes you can apply for AFCAT for the following branches.
1. Flying Branch
2. Ground Duty (Non-Technical) (Administration& Logistics)
The eligibility criteria is given below. Eligibility : (a) Candidates should have mandatory passed with a minimum of 50% marks each in Maths and Physics at 10+2 level and (b) Graduation with minimum three years degree course in any discipline from a recognised University with a minimum of60% marks or equivalent.