According to Jesus Agnostic (more than mythicist) Donini (History of Christianity, p. 143), the enigmatic figure of the kathekon in 2 Thessalonicians would find a solution in the witness of Pliny the Younger:
There was a part of the Roman authority who was reluctant to apply the punitions required by the Jewish and Pagan accusers against the Christians.
So there is no doubt that the kathekon is Rome itself.