You know just enough to be dangerous.
None of your statements are correct.
Cheater cords are not dangerous because they defeat the isolation between the TV and power outlet - there is no isolation between the TV and power outlet. They are dangerous for other reasons (chiefly miswiring).
Autotransformers and variacs are not generally referred to as widowmakers any more than all tree limbs are referred to as widowmakers. The proposed use here is safe and in compliance with the relevant standards.
You are dealing with lethal voltages at the output, no matter if it is ground referenced or not.