B.Com

If you're aiming for a top IIM/non-IIM, CAT is the most preferred option. However, you can also give exams like NMAT, SNAP, XAT, IIFT, which are institute specific & also relatively easier than CAT.

You have opted for B.Com in the UG, then opt for M.Com. Otherwise, you should pursue MBA. Try to finish the course which you have started.

You are eligible for regular MCA if your B.Com is from a recognized university and you had Maths as a subject in 10+2 stage board examination.

No, it will not affect. After B.Com, MBA will add on your qualification and will give you a better career option. Also, if you pursue MBA in finance that will be a career option which will link your B.Com with MBA.

According to me, after completing B.Com, you should opt for banking, finance, and insurance (BFSI) course or finance is a good option. Financial engineering is a growing discipline still it is in the early stage with good career opportunities ahead. These are the fields where B.Com students have edge over others.

You can advised to do 3 years LLB and then do specialized Diploma in direct taxation. With these qualifications you can work with a senior income tax lawyer to get experience and then start your own independent practice.