BCA

No, you can't do both BCA + MCA in parallely. Because BCA is in UG. And after completed your UG (BCA)course with recognised college or university with certified degree, than you can go for MCA. BCA +MCA =5 year course. You can not get both degree parallel in 3 years.

No. As yet, in JNU BCA/B.Tech is not available. Because Jawaharlal Nehru University (JNU) conducts only PG courses now. You can only do courses under PG such as:- MCA and M.Sc etc. from JNU, while you completing your UG course with certified degree. And if you want to study in JNU with UG course BA (H), so one condition is like you must be choose in 10 foreign languages such:- French, German, Korean, Persian, Arabic, Pashto, Chinese, Japanese, Russian and Spanish. Because Jawaharlal Nehru University offers UG courses only in foreign languages right now. These UG courses include BA (Hons).

Yes, you can do it.

Visit for details: http://www.btechadmission.co.in/
No, there is no entrance examination conducted by Christ University for offering admission to BCA course. The candidates will be selected and offered admission through the below mentioned selection process :-
1. Skill Assessment
2. Personal interview
3. Assessment of past performance in Class 10, Class 11/12, during the Personal Interview
Visit for details: http://www.btechadmission.co.in/.

Yes, you face difficulty in placement if companies visited your college. And If you placed then you will not get a decent salary with BCA Degree. Also, Growth is limited. In my opinion, BCA graduates have to do an MCA course then In this case 12th marks become secondary. They SEE the Academics score/CGPA/percentage of your MCA course only. Because They SEE Your coding skill and Development skill at this stage.

No, I am sending you a link. Please read it. BCA stands for Bachelor in Computer Application
link-https://www.shiksha.com/bca-bachelor-of-computer-applications-chp.

I am sending you an authentic link from where you get a list of all good colleges for BCA in Bihar. Please check it out. link-https://www.shiksha.com/it-software/colleges/bca-colleges-Bihar?st[]=103&bc[]=33&ed[]=et_20&rf=filters&ct[]=171&uaf[]=city
But In my opinion, Any Colleges of Patna university will be the Best.

As much as I can understand your question, you are going to pursue 12th this year and apart from BCA, you want suggestions to pursue other courses.
Well I myself is a student of MAKAUT and pursuing BSC in Media Science.
There are plenty of courses to apply for, since your background is Biology, you can go for BSC in Physiotherapy, BAMS, BUMS, BHMS, travel and tourism course, media, fashion designing etc.
Plenty of field is open for you, I'd suggest go where you feel you can do it well.