Interior Design

It is a one year Diploma course.
Fee may vary according to college from INR. 18,000 to INR. 2 lacs.
Eligibility criteria is you have a minimum of INR. 50% marks in class 12th.
You will learn interior design skills with computer added graphic designing and arts.
Here is the list of college.
https://www.shiksha.com/design/interior-design/colleges/colleges-india

You can do it directly.

Yes, it is different and here is the link which has detailed information. https://www.qpractice.com/interior-design-vs-interior-decorating/.

INR. 1 Lacs per year or for the entire course? Check out the colleges for the range of INR. 3 Lacs to INR. 5 Lacs for the entire course:
https://www.shiksha.com/design/interior-design/colleges/b-sc-colleges-Bangalore?ts=87060593&rf=filters&ct[]=278&bc[]=26&fe[]=f4 (Copy paste the entire link on the browser).

Yes, it's wonderful idea it's very useful and it will be helpful in your business

It will be near about INR. 4 lacs to INR. 5 lacs for a four year Bachelor degree course.
It may vary for other courses.
You can see here.
https://www.shiksha.com/design/interior-design/colleges/colleges-india

Hi!
You can only join UG course if you have completed 10+2. Also, there are age bars in most institutes. So, you will have to confirm that too individually from institutes if you have had a gap after completing 10+2.