CBSE 12th

You may face challenges but you can do +2 in PCB provided your school allows (or you may have to appear as external candidate) and you can put extra effort to cover 11 and 12th syllabus in one year.

Candidates appearing for the VITEEE should have secured an aggregate of 60% in physics, chemistry, and mathematics/biology in the qualifying examination (+2/intermediate).
You may refer the link below for more eligibility details:
http://chennai.vit.ac.in/admissions/ug/eligibility

For GATE exam there is no weightage for 12th marks. You should have a B.Tech degree only with a good score to appear for GATE exam.
For the eligibility details refer:
http://www.gate.iisc.ernet.in/?page_id=130

If you want to precede your career in Biotechnology them I will recommend you to go for B.Tech biotech instead of B.Sc because B.Tech has batter prospects in future then B.Sc. and biotechnology is highly in demand in Canada so.
Some colleges in India have admission based on their entrance exam so kindly apply for it if you want your career to be in biotech.

Yes, you can study both maths and biology by selecting AB group in class 11th (science).

I'd suggest against taking up engineering if you're not sure of what you want to do, because it's a rat race out there, and you'll regret it later. I'd suggest you choose between the other two.

There are many opportunities available beyond accounting and management. You may shift to BA and take subjects like economics, psychology journalism rural development and so on. You may opt for 5 years BSL LLB program. You may opt for BA liberal arts. If you have studies mathematics you may opt B.Sc computer science or BCA. If you are interested you may opt company secretary and Bachelor of secretaryship together. You can also opt Bachelor of vocational studies in multiple subjects of your choice. First, decide what so you want to do exactly in your life.

Yes, you will be categorised under all India. You have to make a domicile of Maharashtra for this.

You have less chances because 60% is the minimum requirement of 12th.