"You said that there is no level strictly above Top Secret (just comparmentalizations within that). Is there any actual evidence for that? (I mean, non-classified evidence; I'm not asking you for classified information.)"
There are no levels strictly above Top Secret. At least as far as the military is concerned. SAP programs and compartmentalizations provide more than enough OPSEC to keep things classified. For example, I don't personally have access to the Nuke Codes. But the guys who do all have Top Secret clearances and are read onto some specific compartments and SAPs.
There are also clearance equivalencies at other agencies. DOE for example, has a different kind of system, but they more or less map to DoD clearances. Some agencies are highly compartmentalized, the CIA for example.
"Where did you get the statistic about 1 in 50 to 70 Americans have had at least a Secret classification? That seems absurd."
I don't know the exact number, but I have a pretty good idea that agrees with some back-of-the-brown-paper-bag calculations, it's perfectly within reason.
For example, say there are 1 million people in the military right now. And say all of them have at least a Secret clearance (it's probably more like 70%-80%, most jobs get you at least this clearance level just as a matter of course). That's about 1:300 (or 1:430 or so depending), say right now there are about 25 million veterans (from the Census), that's already 1 in 12.
During the Manning news, it was reported that currently there are more than 4 million people with clearances and one million of them had Top Secret clearances.
http://www.dailymail.co.uk/news/article-2041326/More-4-MILLI...
That pegs it at 1:75 right now with Secret levels and 1:300 with Top Secrets.
1:50-1:70 for people who do or have had access to classified material is is extremely conservative.