Diploma

Since you are working, you can pursue a part time engineering. Or pursue a BA/B.Sc. (IT) to get a degree. Or else, gain some experience and then later pursue MBA.

I would suggest pursuing B.Tech because it has more opportunities and is overall better than BCA. But, you are eligible to pursue BCA and if you want you can pursue it through lateral entry.

You will have to clear your UG to pursue any PG course. You can pursue Diploma based on class 10th, but that is not recommended.

Eligibility is aggregate of 60% in the qualifying examination. Minimum of one year work experience after acquiring relevant qualification. Should be employed in or around Chennai. Duration of the course is 4 years.
The course fee is 1.5 lakhs.

Yes, you can apply just check out if admissions are open.

For one year PG Diploma course at Apeejay Institute of Design, New Delhi is approx. 100,000 per annum which can be paid quarterly or half yearly.

Merchandiser, stylist, content writer, blogger, assistant designer, fashion illustrator, etc.

The decision would be based on your interest areas and long term career plans. If you want to pursue a career in academics / technical / research areas then M.Com would be a better option. However if you want to move into general management / operations side then MBA would be better. An MBA is always better that a P G Diploma.