Science

You may refer the links below for the list of B.Sc colleges in Thane:
https://www.shiksha.com/science/colleges/b-sc-colleges-thane

Yes, you can attend exams like above you mentioned but it is compulsory to get 75% in your 12th board and 65% if ST/SC candidate.

Yes, there are many colleges in Agriculture in Mumbai:
https://www.shiksha.com/science/colleges/b-sc-agriculture-colleges-maharashtra

No, you need to have maths with physics and chemistry as your stream in intermediate or 12th board to join in any B.Tech course.

Most MBAs are generalist even if they have a subspecialty attached. The M.Sc in (a business area) is meant to be more specialized. Many employers will not differentiate which is better. They'll often say MBA or equivalent.
So, it depends if you want a generalist orientation (the MBA), or a more narrowly defined experience (the M.Sc in a business area).

Be it for engineering / medical courses, you need to clear the minimum cutoff as set by the specific college to get admitted in the particular course/branch. So, say if you get 50/200 you may not get ECE / CS in any Mumbai based colleges.