I do have a comment that I hope may be helpful. In the article to which this thread applies, "Manipulating the Good News", the authors discuss the publication by Roman authorities of the "Acts of Pilate", as part of the Roman Government's efforts to demonstrate that Jesus was indeed a criminal disserving capital punishment. Although Josephus' works may well have been altered in many ways, it is quite a stretch to hypothesize that Pilate's governorship of Judea commenced as early as 21 AD. For one thing, the synoptic gospels include references to Pilate. At least Mark and Matthew were published close enough to the events so that many would have remembered details of the accounts of direct witnesses. Misnaming of the governor under these circumstances would not have withstood attack.
But there is a more obvious explanation for the discrepancy observed by Eusebius. Although your analysis indicates that, "…these documents date Jesus' trial and crucifixion to 21 AD…", that is not exactly correct. Of course the Roman documents were identified with reference to the reign of the Emperor. The year we call "21 AD" was TIBERIVS IV, that is, the fourth year of Tiberius' consulship, being the seventh year of his rule. Pilate commenced his governorship sometime around TIBERIVS IX or the ninth year of Tiberius, or 26 AD. The traditional year of the crucifixion is 33 AD. If it occurred instead in 31 AD, that would have been TIBERIVS XIV, a very easy slip of the quill to go from there to TIBERIVS IV. My hunch is, and common sense suggests, that Maximinus II Daia released the true records of Pilate, and the copyist just made a slight error and dropped an X from the header. The more telling observation is that Eusebius' best and only shot at a criticism--of what could only have been persuasive evidence of criminality or insurrection considering its release by a government seeking to stem the tide of Christianity--was to fixate on this obvious transcription error. Additional evidence of the significance of Pilate's account, is that once the Christians gained power, the Acts quickly disappeared from the scene, as did any other references from which its content could be directly inferred.