BSEB

Mathematics can be the good option. Because Maths is compulsory for pursuing Computer Science and application courses.

You have not write about the category and sub category you belongs. However if you belongs to general category and VJ subcategory and domicile of Maharashtra than you can get the B.E. Computer Engineering in asked college.

Yes you can take. The eligibility criteria is given below.
1. Applicants must have passed their Intermediate standard from the recognized board with the minimum score up of 50% in Physics, Chemistry, mathematics and English.
2. Candidates appearing in qualifying exam are also eligible to apply for AMUEEE 2020.
3. Candidate who have cleared diploma in engineering programs from AMU with the minimum of 50% are also eligible to apply for this entrance.

You can apply for DU but there are very less chances to get a good college.

You can check it by clicking here :------
https://www.shiksha.com/college-predictor

Yes, you can get admission in DU with these percentage in class 12th. In DU, your top four subjects of class 12th are considered. You will be able to get the college in 2nd and 4th cutoff list.

The link below gives you the list of top BCA colleges in India:
https://www.shiksha.com/it-software/ranking/top-bca-colleges-in-india/96-2-0-0-0
You may filter the list and can view the government colleges from it. All the best.

No, it'll be quite hard for you to get admissions there. You have to score a better marks in your JEE Mains. If you have any query do let me know in the comments down below. All the best.