As per S.399(3) of Criminal procedure an application for revision can be made by or "on behalf of any person".

Would it be therefore correct to say that a relative or a friend of the accused can file revision application on behalf of the accused?

In such case would it be correct to say that no authority of the accused is required by such relative or friend to file revisions application on behalf of the accused especially when the law itself has authorised filing of an application on behalf of any person?

Can an advocate engaged by an accused file a revision application on behalf of the accused invoking the said provision especially when the accused is not available  within the jursidcition of the court to sign the application?

I agree with your interpretaion.