Design

If your interests lie in MBA interior designing you should definitely pursue a course in interior designing. Scope in interior designing is immense, and students go on to make lucrative careers after completing their course in interior designing. For now, you should focus more on your course in B.Sc (Home Science), and make plans for the future side by side.

You seem to have already decided the course you wish to pursue and that is fashion designing. You may pursue the same course from either NID or NIFT.

No. See the reason NID doesn't prescribe a syllabus and changes its pattern every year is to discourage this recipe style of reaching solutions.
Doing coaching is like walking on a rope bridge you loose freedom.
Stay original. Keep observing. Sketch everything, for drawings, speak always speak the truth. Watch anime. Keep doodling.

If you wish him to get a good career then you should ask him to complete 12th with good grades and prepare for entrance tests after it. The best institutes are NIFT or NID, you can send him in any of the institutes.

M.Sc fashion designing eligibility
- The candidates should complete their B.Sc. degree under any registered University with Fashion Designing subjects with 50% or above marks.
- The marks of admission for this course vary from university to university. In various colleges and Universities, it conducts entrance exams plus interview for the admission of the students.

Refer to the link below to see the part time colleges in Mumbai for Fashion Designing:
https://goo.gl/qXXa91

Yes, you can prepare for both simultaneously. As without sketching, the entrance will be difficult to clear.

There is no university which provides lateral entry for a correspondence course. You can check the same from the college you wish to take admission in if they can allow you.

Refer to the link below:
NIFT : http://nift.ac.in/scholarship.html
NID : http://admissions.nid.edu/faq.aspx#fas