Hi Farhan,
I am no expert in Certs but have worked with a few and these are my findings.
Please see the answers below to your questions.
1 - The answer is yes. We have done this. We integrated to a 3rd party system and they only had Development and Production systems. So for testing we implemented the same cert on our Dev and QA PI systems.
2. No it is not. Not 100% sure why, but some 3rd parties gives us a cert and if we install it the interface works fine without a CSR(I might be wrong)
3. We normally use X.509 Certificate.
4. Will have to test it. But one should work
5. NWA -> Certificates -> Key Storage -> TrustedCA
6. Only on Java because it is only the java stack making the connection. If you go through the ABAP stack you would have needed to load the cert in STRUST.
Hope this helps.
Regards,
Jannus Botha