Design

While there are associate degree programs available, entry-level jobs in advertising typically require a bachelor's degree. A four-year advertising degree program entails both the arts and sciences, combining courses in visual design and copywriting with those in communication, psychology and market

The best textile designing college in Rajasthan is NIFT Jodhpur.

Yes, you can. Unless otherwise mentioned by the university.

It will depend on the schools' requirements where you intend to go to. You may need to do architecture first to do a Masters. It won't help that much to do industrial design, but if you want to architectural projects then it could be useful.

Assume you must be nervous, being in an entirely different field how you would catch-up without any basic knowledge. If so, then I would suggest to go for a post graduation programme than Diploma, where you get to cover all the basics in first semester. Speaking of courses, I would suggest fashion d

Yes you are eligible for graphics designing.

There are plenty and I repeat plenty of institutions across India for such courses. https://www.shiksha.com/design/communication-design/colleges/graphic-design-colleges-india

Yes, it is also a good choice. You can join BTech in Automobile Engineering after class 12th under which you can work on car designs.

For which region and university are you asking for?

No Diploma is equivalent to graduation, one should go for degree programs. Then only you can do advance studies eg: PG and Ph.D.