Also, was it ever said that the Admiral's assistant was a full Betazoid and not 3/4, or 1/2, 1/4 or any other fraction?
All that we know is that Sabin was Betazoid. He could have been only partly that, of course. It would explain a great deal as to why he didn't simply scan Tarses himself. But we don't know for sure how 'pure' a Betazoid Sabin was. If he was anything other than full Betazoid, they never said so.