Hi, here are answers to your questions.
1. Yes ISTQB certification is good for testers, Be it for freshers or experienced. Recruiters pay a lot of attention to the certifications. Just don't go on telling about certification to the person who interview you because in testing industry elite people hate certifications. Certifications are only for recruiters to notice your skills. There are many certification options ISTQB, CSTE, IST, BCSQA etc.
2. Testing is the industry where you have to work from the scratch. IT or non IT doesn't matter. You start from manual testing and then get into automation. You can do course at NIIT or any other institution. I suggest doing online course instead of spending thousands at NIIT. Even Udemy or Pluralsight or youtube can be a good source to learn manual testing. I learned from institute and later found out that all they did was ran few slides in manual testing lectures. I got more knowledge online instead of institute.
3. Certifications are controversial in the testing industry. Some elite and authoritian people hate it. And on the other hand recruiters expect students to have it. So it is upto you. I didn't took risk and got myself ISTQB and CSTE.
4. No college or university has PG courses in testing. But most of the training institutes have them. So it is upto you to choose them. My suggestion find Udemy course on automation and do it from them instead of wasting a lot of money on institutes. Also do the manual testing course through youtube. Because most of the institutes just run powerpoint slides for showing concepts of manual testing.
5. Do the manual testing course first and then give exam. Because that way you'd be able to sink in those concepts.
6. You get the manual testing job at first in any company for testing job. You don't get automation job directly. Only game testers don't need manual testing job at first. But their platform is different and they don't test web or desktop applications. So your path for job is something like this - manual testing and then automation testing. if you know automation and company has opening for fresher in automation then you can also skip the manual testing job and get into automation directly. Usually performance testing jobs and web automation in this category. Not many companies risk training time for automation position like this but this is also possible for freshers. Demand for manual and automation depends on the company. UX and UI designers prefer manual testing. SDET based jobs require Automation experience.
I hope this helps. Also do note that if you attend off campus of most of the companies and they also have testing opening for non IT background. You can get in through that mode as well.