Hey Akash,
You can definitely do your graduation in CSE. Only thing is start working hard from the first day itself, learn C, C++, Java, Python and as many languages as you can during your semester breaks or whenever you are free.
You can also learn them for free from an app called LearnVern, download it and start learning.
I am learning Free courses on www.LearnVern.com I highly recommend you to check it out. Or Check the Android App  https://goo.gl/1MRPF4 I got a Free Certificate as well.

And if you have the desire to achieve anything you definitely will achieve.
All the best buddy

Yes you can just give and manage your time well.

Yes you can apply for cse with PCMB.
They only consider your PCM marks with the competitive exam marks. Your Biology subject mark are not considered. If your interested in computer science then you can apply for Cse in engineering.

yes, a Biology Student can join B-tech Computer Science Engineering there is no Restriction:

*Biology student have the option of selecting either Medical or Engineering that's the added benefit of the course, since Maths, Physics, Chemistry is already there along with Biology which is also needed for Engineering course.

*So if you are interested in doing Computer Science course and feel that you have not scored good marks to apply for medical then you can go ahead and apply for Computer Science course in some best college.

*Try to apply for AIEEE exam and apply for the good colleges with the AIEEE marks.

*Any more queries, feel free to leave a post in our forum..