Class 11

Vast difference, Maharashtra board's syllabus is merely an introduction of the topic, no background history, no further application whereas CBSE board uses a step by step approach for any topic.
You will see that all the government exams have their syllabus based on NCERT books (i.e. CBSE books), that is because it gives right amount of knowledge to the child at the right time.
More importantly, the language usage of CBSE books is far easier to decipher as compared to that of Maharashtra State Board.

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

It is not possible to pursue any engineering degree while studying in classes 11 and 12. You need to complete 12th class with a good score to do engineering.
To pursue genetic engineering you should take PCB and you are advised to write some entrance exams like JEE Mains, VIT , SRM, etc. in order to get a good college for biotechnology and genetic engineering.

CBSE is a better option after 10th board in comparison to state boards because it's syllabus is aligned with majority of medical and engineering entrance examinations.

The scope is huge. But you should be able to cope up with the study burden and you will be questioned about it in every job interview so you should be able to justify your choice.

You are already heavily loaded because of Physics, Chemistry, Maths and Biology. Why do you want to take additional load by taking computer science? You are eligible for seeking admission in B.Tech CSE or IT even without computer science as an additional subject in 10+2, but if you are confident of taking load please go ahead with CS as an additional subject.

You should opt for arts stream in class 11th as it contains topics like general awareness which have a great importance in the media field.

You can opt any subject you want because for NIFT entrance you need GK, English, Maths (till class 10) and Logic Reasoning. So your class 11 & 12 stream won't matter.

It depends on your class 10th results. Also, it depends on school to school and the stream offered by them.

Just do some simple things:
1.Go to your class daily.
2.Study 6-7Hour daily
3.Leave Social Networking Sites.
4.Contact with good students
5.Practice mathematics daily
6. regularly revises your organic and inorganic chemistry part.
Rest are same.
All the best.