B.Com

There is nothing as minimum marks. You give entrance exam like everyone else, then the names get shortlisted depending on the number of seats. Let's say B.A. Economics has 60 seats. Then, the first 60 rankers will get it (including quota). So, no one can predict what would be the least score which converts the seat.

It's going to be tough. You can try somewhere else or apply next year.

Best colleges that offer B.Com are:
-Narsee Monjee College of Commerce and Economics (Mumbai)
-Symbiosis College (Pune)
-Mithibai College (Mumbai)
-R. A. Podar College of Commerce and Economics (Mumbai)

http://www.nifm.ac.in/
Here is the link to the official website where you can find all the information you need.

You will have to first complete your graduation with more than 50% marks. Only post it, can you purse MBA as MBA is a Masters program. If you find it difficult to complete B.Com then you can try your BA as an external student and complete it via correspondence in 2 years' time.

Since you are giving us options to choose between B.Com (Hons) or BBA so I'm assuming that you are interested in both of the courses equally. Look, Delhi University is well reputed, recognised and everyone gives first priority to DU then comes IP University. ARSD of DU is one of the reputed colleges which is perfect in every aspect.

These are some top colleges for B.Ed in Delhi:
Lady Irwin College
Indira Gandhi National Open University (IGNOU)
Army Institute of Education (AIE)
Guru Gobind Singh Indraprastha University
Siddhi Vinayak College of Science and Higher Education

Yes, there are chances as this year colleges are out with most realist cutoffs. Look for other colleges' cutoff also.

A period of 4-6 months should be sufficient for CAT. Over this time one needs to put in 3-4 hours of daily sincere effort. Self-study and joining of classes for preparation with a lot of doing of mock tests will help you to excel in CAT.