IIM Ahmedabad - Indian Institute of Management

It is not something one can answer for you. It's something you need to discover for yourself. Rule of thumb: If you want to stick with the technical line then M.Tech. If you are more interested in management then MBA. Refer to this TOI article to make a more informed decision:
http://timesofindia.in

No MBA in mergers and acquisitions is not offered by IIM Ahmedabad. IIM A only offers degree in general management. Having said that, you can always take electives regarding mergers and acquisition in 2nd year, if you have particular interest in the said topic.

Yes, its worth every penny. The level of faculty, interaction with peer group and learning is unparalleled in India. Hence, its worth it.
Do understand that unlike other government institutes (read IITs/NITs), IIM's do not take any form of monetary support from government. Hence, the complete expens

You can start preparing diligently for CAT. If you do not have much idea about CAT, I would suggest you to join a coaching institute with good faculty to help you get the basics right in DILR/Verbal/Quant. After the basics the next challenge is to solve the said question within a stipulated time. He

Sorry to say but your prior academic performance makes it very difficult for you to get a call from IIM A. Having said that, do search for colleges which do not give weightage to previous academics.

Since you have completed a 3-year degree in 5 years, I am assuming you have not-that-great academics. If that's the case then getting admission to IIM-A will be difficult even after getting a high percentile as IIM-A gives substantial weightage to academics.

Please refer the selection criteria of IIM A- PGP: https://www.shiksha.com/iim-ahmedabad-announces-the-shortlisting-criteria-for-pgp-batch-2016-18-article-11459-1

Join any CAT coaching center and get your basics cleared. After going through the basic reading materials, you can graduate to more advanced practice by going for more advanced time based sectional mocks. Do understand that CAT is a test against time also. Just having the right know how is not enoug

I would suggest you to go for general MBA which is offered by all premier B-schools and not concentrate on any specialization initially. You get to choose your specialization in final year based on your interest.

Prepare well for CAT. Hard work is the only way to get a decent percentile in CAT. I would suggest you to join a coaching center and religiously follow their guidelines if you are not self motivated enough. Otherwise, get your basics right and then go for mocks for practice.