Diploma

After Diploma, you can only do Post Diploma. Post Graduate Diploma is done after the Under Graduation. Also, Graduate Diploma is just Diploma and not a degree.

First, if you have completed your 3 years diploma in mechanical engineering, then you are eligible for various state and central government jobs. Keep a close watch on leading job portals and employment news.
Second, for boiler operation engineer, you must meet the following criteria-
i) 3 years diploma or BE/Btech in Mechanical, Electrical, Chemical, Instrumentation, Power Plant and Production Engineering.
ii) Candidates should have experience in Operation and/or Maintenance of Boiler for
a) Not less than two years in case of Degree holders
b) Not less than Five years in case of Diploma holders

Sorry to say, but you are not eligible to take admission in this college with these marks.

No, it totally depends on the college or university norms.

There is nothing tough and both the courses are good but I think you should choose M.Pharma because with M.Pharmacy, you can get better job opportunities, good position and obviously good pay scale. After Masters, you can also pursue your PhD if you want.

No, you can't change your branch. You will have to take admission in your relevant engineering branch.

Please name the college or university where you want to take admission so that I can give you full details of those colleges.

Yes, it is. It is also recognised by AICTE.

I am listing some top colleges. Do check the list below-
IIFT North Delhi Campus - Fashion Designing
International Institute of Fashion Technology-
Fashionista School
JD Institute Of Fashion Technology, Dwarka
International College of Fashion
Apeejay Institute of Design