Hess was not locked up in solitary; in Spandau prison, kept by the allies for the imprisonment of the highest-level war criminals, Hess was in what we would call "general population" and could socialize with the others. In later years (I think, after Albert Speer and Baldur von Shirach were released in 1966), he was alone, only because he had a life sentence and everyone else had been released.
Source: Spandau: The Secret Diaries, by Albert Speer. I can't pull quotes from it just now as it is out on loan.
You’re right.
Fantastic book, and you beat me to that one. Yeah, Speer spent no less than 20 years in close proximity to Hess and even after all that time didn't quite figure him out. I think his best guess was that Hess was a crazy guy pretending to be a sane guy who was pretending to be a crazy guy. The Brits certainly thought so.
As for this thesis, color me skeptical but not dismissive. If it were me, Hess wouldn't be my first choice for a mission of this delicacy. But Hitler was Hitler, and not always the best judge of his own staff - according to Schirach he thought Ribbentrop was a genius on the order of Bismark, for example - so I wouldn't rule it out.