Computer Science and Engineering

Most of the colleges offer lateral entry admission to Diploma students.
http://www.rscoe.org/admission.html

You can take the help of YouTube. You will find video tutorials which will help you.

SRM will be a good option.

Yes, mobiles are allowed in Kongu University, not even in hostel, mobiles are allowed even in the college and classes also.

See, if you are interested in both. Then, it will cause a problem for you. I'll prefer you to go and discuss each and every aspect with your elders and the person who is in this field, which your are confused in. As far as I have seen opting for the GATE is better for you as there is job security fo

Which college? Name a college and I will return with fee details.

See, if you want to go for the research, then you can opt for the chemical engineering. Otherwise, opt for the CS and there is a wide scope of both the streams. So, for the future in both, the streams you only have to look for your passion.

You can apply for exam in the last year. But, you should start preparation as soon as possible. You can start it from right the moment now so that you will cover lots of knowledge. Hope this will help you.
Good luck.