Design

You can do one year diploma in fashion or 2 yrs advance diploma in fashion designing from know institute like NIFT /NID /International school of design Kandivali, etc. This course will definitely provide you good placements.

Yes, surely, you can do diploma and even you can do MBA or M.Sc in fashion designing after your B.Com.

Yes of course, you can do fashion/interior degree/diploma courses simultaneously with your B.Com. So, by the time you are graduate, you will get both the degree and then pursue towards your field of interest.
All the best.

I assume that your wife has not completed 12th exams. So, one needs to complete 12th boards before pursuing any course in any University.

National Institute of Computer Arts offers only Diploma course which can be pursued after 10+2. So, have a look on the courses awarded and decide on the Diploma which you wish to pursue.
Link: https://www.shiksha.com/college/national-institute-of-computer-arts-dadar-west-mumbai-25514

Yes, it's possible.

Get all the required information from the below link:
https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.shiksha.com/architecture-planning/course/amp/b-arch-mit-school-of-architecture-mit-art-design-and-technology-university-loni-kalbhor-pune-275930&ved=2ahUKEwj_yJfzxKjcAhVJLo8KHdKqDR0Q

Hi Aakanksha, NIFT also provides regular graduation courses only. You cannot pursue two regular graduation courses together. It is prohibited by law.