Agree strongly with all your excellent points up to here. As a statistician I'd argue that what the scan-radiation risk (very low) actually needs to be compared to is is the terrorist-attack risk (extremely low); only if the former is a lot lower than the latter does the scanning even theoretically improve one's safety.
The following (pretty funny) cartoon includes the point that the risks are about the same, so scanning can't actually be of any safety benefit. Who actually benefits (following the money) are people like Michael Chertoff, Bush's Secretary of Homeland Security, among whose clients now are Rapiscan (maker of one brand of scanners). So the people who think this increases security have, as usual, been duped.
YouTube - TSA Enhanced Screening Procedures Explained