M.Tech

Yes, M.Tech can be done with any stream irrespective of your B.Tech stream.

Yes, why not.
You are eligible to leave after 4 years with B.Tech degree or you will continue M.Tech for coming 2 years.

No difference in the package. Infact, you may get more than B.Tech Comp Science grads, as a few companies may treat you as Post Graduate. Once you are an engineer or in the process of becoming one. You are free to do any job. Ultimately what differentiates you are the skills you gain and hone over and over again.
Additional Information: Predict engineering colleges you can get on the basis of your JEE Rank: https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor?utm_source=cconnect&utm_medium=CR_JEECP&utm_campaign=8321427.
All the best!

There are over 6 M.Tech in Biotechnology Engineering colleges in Bangalore. Please refer to the link provided below for a detailed list of these colleges. You should be able to navigate to the top colleges in the list using the filters on the web page. Regards. https://www.shiksha.com/engineering/colleges/m-tech-biotechnology-engineering-colleges-bangalore.

Yes, you may get any autonomous college for chemical engineering with this score.

Yes, you can change your engineering branch in M.Tech.

Please check for your eligibility to appear for UGC NET from the criteria mentioned at the link provided below. You will have to opt for engineering sciences as your subject in the NET exam. Regards.
https://www.shiksha.com/teaching-education/exams/ugc-net.

You have so much time for GATE exam. Even if you want to give it when you are in the 3rd year of your graduation then also you have 2 years. Download the syllabus of GATE for ECE students and start studying and for help join some online coaching like that of Ravindra Babu. It will provide you with the study material along with the necessary guidance.