Herbert FrahmI think that the person we are looking for is someone who is in the room when the important discussions are going on and has access to the most sensitive documents.
IOTL around this time Willy Brandt (Herbert From) had an aide who was in the Stassi, and it was a big surprise as he passed several deep background checks.
And yes, that whole affair grew into what it did because of the sheer shock value of the whole affair and the embarrassment it did to the FRG, as well as the SPD specifically (just handed the CDU ammunition to say that the SPD was in bed with the DDR). I doubt it's the same person, among other things even if the CIA was on the top of their game, they wouldn't do as well at this as the Stasi, simply because they'd lack the huge cultural background that allowed the Stasi to be so effective at infiltrating the FRG (and presumably, the FRG to infiltrate the DDR, but those records are still sealed so that's speculation).
I agree with you and Salamander with regards to what Hillebrand should do and what the real threat is. But could be some stupid little thing, or some other strange happen stance, keeps him from being able to do what you suggest, and only dumb luck that the CIA haven't blathered to the wrong person yet.The main problem is that there is no way in hell that the American Ambassador should have that information, and it is to his credit that he understands that.
What the Ambassador should do is take a "quiet" personal trip back home and inform the CIA director one on one that too many people have the information on who the asset is and getting them transferred quietly to other less sensitive posts including himself.
BTW Ambassador Hillenbrad was at this time the IOTL Ambassador to the FRG, also he was one of those professional career State Department officials who served the President no matter what party they were in.