B.Tech

Yes, you can apply for B.Tech agriculture as this subject is optional subject so it will not make you a field-specific person, you can apply for this course.

If you ask for a straightforward answer, I'd say it makes a lot of sense to startup while in college. Mostly college allows.

Look the criteria for fees is different for different colleges, But you must have 50%+ to pursue CSE after Diploma for B.Tech.

Yes, anyway The weightage of 12th standard's percentage matters the most. A minimum of 50% is a must.

Best colleges for B.Tech after Diploma are-
Guru Nanak Dev Engineering College, Ludhiana
DAV Institute of Engineering and Technology, Jalandhar
Government College of Engineering, Pune
Guru Teg Bahadur, Delhi.

You can find sample papers for B.Tech on Shiksha.com. This site provides each and every possible and important question papers.

According to previous years cut off's, you may or may not get admission as it's totally depend upon the cut off of the year you are appearing in.

The university has two institutions offering Computer Science related courses. One is MMEC, the engineering college offering CSE BTech. The other is MMICTBM which has a Software Development course.
Their blog has some insights. I shall suggest putting a query to them through the admission helpline and see if you get the info what you need.
https://blog.mmumullana.org/5-very-good-reasons-to-study-software-development-btech-cse-program-at-mmdu/

Hello Shital,
It is very nice to see that you are interested in doing this course in spite of being a female student. Of course this course is open to both the sexes but participation of girls is quite low. I am attaching a link that will give you all the required information about different aspects of admission in this institution.http://www.manetpune.edu.in/BSc-Nautical-Science/Eligibility-Criteria
Best wishes to you.

You need to appear for IMUCET.
Interested candidates can apply online/ offline with required information. Candidate has to appear for the IMUCET (Indian Maritime University Common Entrance Test). Link:https://www.shiksha.com/b-tech/course/b-tech-in-marine-engineering-lateral-entry-himt-college-kilpauk-chennai-268542