Can an institute without any affiliation to any university whatsoever award a post graduate advanced diploma?

There are certainly specialized institutes of study that can do this. There's no law saying that only universities can offer postgraduate courses of study and award diplomas. In order for the dipomas to carry any weight, the institute would have to be accredited or recognized by the relevant professional organizations, but I know of some institutes and special programs in a couple of fields that grant diplomas respected within their fields.