CBSE 10th

A few good institutes which you can consider for doing Diploma are:National Institute of Fashion Technology
National Institute of Design
Lovely Professional University
National Institute Of Interior and Fashion Technology
Institute of Apparel Management
International School of Design
Indian Institute of Fashion Technology
LISAA School of Design
Vogart Institute of Design and Management, Delhi. Pearl Academy.

10+2 is the basic qualification for pursuing most of the PR and journalism courses. There are certificate courses which don't have credibility. I would not advise you to go for those certificate courses.

It depends upon the institution you are looking for but majority they ask for higher secondary education that is 12th class so I think explore and consult then choose. All the best.

Your profile seems decent.
For PGDM 2020 Intake, Great Lakes Institute of Management will accept scores of GMAT taken after 1st January 2018, CAT taken in 2018/2019, XAT taken in 2019/2020 or CMAT taken in 2019/2020.

Applicants are evaluated based on GMAT/CAT/XAT/CMAT scores, past academic performance, essays, work experience (if any) and performance in the personal interview. The selection process builds in a criterion that facilitates evaluating each applicant on Academic Abilities, Standardized Test Scores, Rank Holders, Integrity & Ethical Values and Diversity of Background. At Great Lakes, we do not have fixed cut-offs, we try to evaluate candidates in a holistic manner. We consider the complete profile, so while the test scores are important, we emphasize more on selecting well-rounded candidates with a strong overall track record.

It is great to have experience since it will add more value to your resume when it comes to you MBA placements. Marketing management will be good option for you as per your experience.

FMS features among the top 50 institutes, so competition is tough. To be on the safer side you should try and score more than 90 percentile in CAT.

Admissions for full-time MBA/PGDM courses in reputed colleges across India are given only to those students who have scored at least 50% marks in their graduation.

No problem, try again and complete your engineering.