This any one thing has been. Those multiple things have been.
Have been is present tense and had been is past tense.
Yes.Am, are, is, was, were, has been, have been, had been are all linking verbs.For more information, please see "Sources and Related" links.
It depends on the context. For example,You could say " I have been approved to be a banana."It wouldn't make sense if you said "I has been approved."And you could only say has in " It has been...
have been installed means now and had been installed means past