BCA

The following are the good colleges in Delhi for pursuing BCA and MCA courses. The duration of BCA and MCA courses is three years. The fee structure may be varied according to Institutes, So please visit respective Institute for more details.
1. IITM Janakpuri (BCA)
2. Vivekananda Institute of Professional Studies (VIPS), New Delhi (BCA& MCA)
3. CPJ College of Higher Studies & School of Law, New Delhi
4. Fairfield Institute of Management and Technology, New Delhi.

Yes, you can apply for the MCA course at IGNOU. If meeting the eligibility criteria as given below.
Eligibility: Any Bachelors's degree of minimum 3(three) year duration from a recognized University AND Mathematics as one of the subjects at 10+2 level or graduation level; else the student is required to pursue and pass the BCS-012 course concurrently with IGNOU MCA.
If you had studied Maths at +2 or graduation level, then you need not register for BCS-012.

Yes, you can apply in M.Sc in Computer Science.

I am not sure if that is possible because even after 4th semester you would have taken such decision it might have been possible at the end of 2nd year however now you are in your final year that too 1semester completed so which means your practicals and projects must have been recorded by your institute and other important aspects like placement etc. Suppose even by some means if it's possible I will strongly suggest you not to take admission in any other institution. It will not be easy to adjust. I hope this helps.