It is fascinating stuff, and certainly worthy of consideration. I have no doubt that Mahan could have found "something" at a Greek library in Constantinople. But whether he had the scholarly chops to discern a fraudulent work from a legitimate one is unknown, at least by me.
Here is a link to a translation of the so-called "Letter of Pilate." It doesn't provide much new information and has many of the hallmarks of a later fraud.
This article includes some of the authentic ancient sources that tell us a little more about Pilate, namely Josephus and Philo.
There is a ton more, including the so-called
"Acts of Pilate" and
"The Report of Pilate the Procurator" and
The Death of Pontius Pilate" among others. All of these works have been considered "historical fiction" of later dates by most Church scholars.
Thanks for extra info.
After Apostle Paul was arrested, a descendant of King Herod the Great (the "King" who feared the birth of Jesus):
Julia Drusilla, married to Antonius Felix (procurator of Judea)
was an observer, when Governor Antonius Felix heard the charges against Apostle Paul:
Paul presented the gospel as part of his defense, but Felix delayed giving a verdict. Some days later, Felix with his wife, Drusilla, summoned Paul for another hearing.
Speaking before Felix and Drusilla, Paul "spoke about faith in Christ Jesus . . . righteousness, self-control and the judgment to come" (Acts 24:24–25).
- https://www.gotquestions.org/Drusilla-in-the-Bible.html
Her son, Agrippa III - aka Marcus Antonius Agrippa - died in the Mt. Vesuvius eruption (79 AD).