PG Diploma

For which course do you want to apply there?
As you are a graduate, so you can apply for postgraduate courses according to the eligibility criteria of that course.

Yes, you can do that, unless mentioned otherwise by the institution.

You are eligible to apply for this course.

Yes, you can apply for admission after completion of your degree. You are eligible for the course.
Good luck.

Yes, you are eligible for PG Diploma. You can apply for admission.
Good luck.

PG Diploma in law is only possible when you have a UG degree and mostly Diploma courses are not available. Best visit the website of university and download brochure/prospectus of course.

The admission eligibility criteria for PGDFSST program at AIISH is as under.
Eligibility: Candidates who have passed in any of the following examinations of any recognized University/ institute with a minimum of pass class in the qualifying examination are eligible to apply.
a) B.Sc. with Physics/Mathematics/Electronics /Computer Science/Forensic Science/ Speech and Hearing as a major component at B.Sc / B.ASLP level.
b) B.E /B.Tech with Electronics/Computer Science/Communication
c) M.B.B.S.
Hence B.Sc Life science students are not eligible for this program.

As of now, there is no specific PG Diploma offered by IGNOU related to physics. So you can go for any PG Diploma as per your choice and interest. Also, remember your potential while choosing because there are numerous examples. If talking about the scope I would suggest journalism and Mass Communication and Information Security have good job perspectives. Good Luck.