I think that we can be sure that the TF as we have it now is NOT what Josephus wrote. It is obviously Christian ("if it be lawful to call him a man" ?) - Josephus was not a Christian - and there is no record of it before Eusebius, even though other Christian writers were aware of Josephus and there's no reason to believe that they would pass on mentioning something so obviously helpful to them.
(The idea that the Christian parts were interpolated rather than the entire passage being a fake is still possible, possibly by a scribe mistakenly including marginal notes. I do, however, wonder why - if that is the whole story - the passage was not quoted earlier, although this doesn't mean that the passage did not exist in some form.)
Still it's a jump to nail Eusebius as having forged the passage. It's a neat solution but reality often isn't that neat and tidy.
Edited by PaulK, : No reason given.