Sardar Vallabhbhai National Institute of Technology, Surat

Refer this link for the eligibility criteria for admission:
https://www.shiksha.com/engineering/course/m-tech-in-environment-engineering-svnit-sardar-vallabhbhai-national-institute-of-technology-surat-358917

Please visit this site first
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor.

No, you can not take admission in SVNIT without appearing in JEE-Main. All the B.Tech admission in NITs are done on the basis of ranks obtained in JEE-Main. However, you are entitled for 5% reservation quota in your category for admission.

Yes, it is good score to get admission there. You just have to have more than 75 % in your class 12th.

If you are applying through the general category, then it may be difficult to get in SPA. You may get admission in NIT.

Choose the branch of your choice with decent college if not the top one because branch is what you make a career in. If you choose any branch you might not be able to score well in that, as that is not your interest area.

Yes, you have a great chance of getting NIT-Trichy.

M.Sc admission is not included in JEE. You need to appear for the entrance test after B.Sc. But you can opt for dual degree program if you are interested in (B.Sc +M.Sc) from SVNIT itself on the basis of JEE rank.
Best of luck.

Yes, there is strong possibility of getting a good NIT for your category score. You should be calm and choose the list if colleges wisely and go till last round of counseling it will be helpful. You can check out this link for prediction of colleges. https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor.