M.Tech

Hi,
In terms of requirement of engineers - industry requires them in two levels. In junior level, they recruit Diploma engineer and B.Tech/ BE for senior levels. As you are in the mind set of job, your B.Tech degree is strong enough to fetch you a job in equivalent level.

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.indiatimes.com/home/education/news/MBA-vs-MTech/articleshow/21908011.cms

You must keep your basic concepts clear of computer science subjects. You can go through previous years question papers to get a feel of toughness and difficulty of GATE.
Manipal entrance examination is an easy exam to crack, if you are through with subject knowledge and good in applying them. You can refer to study materials available in market by Made Easy or you can subscribe for material from Career Launcher to practice questions.

Nowadays, most of the universities allow B.Tech only for M.Tech.

You can do courses in website development along with database skills and java programming. This course will help you in getting a good job.

It completely depends upon your inclination and liking. If you want to rise high in managerial positions and administrative positions in a corporate sector then you must opt for MBA. But, if you are more interested in technology and development or in lecturer job then you must opt for M.Tech.

You are advised to review and evaluate MTech Software Systems from BITS Pilani WILP. It is delivered by distance learning and I am of the opinion will meet your requirements.

After completing ECE, you can pursue your Master's degree in power electronics, VLSI design, electronics PCB, telecommunication, etc.

No, you cannot. You have to pursue B.Tech to pursue M.Tech. After completing BCA, you can go for MCA.