It's not a question of need. The urgent interest in Origen's summary of Josephus's account is to authenticate, if possible, that Josephus wrote that defendant James's brother was Jesus "called Christ." Authentication by Origen is only possible under an assumption of Origen's good faith.I don't see the need to think Origen misremembered the passage, if it might be him reading what "really happened" from Josephus's description.
Thus, the interesting assumption for potential authentication is that Origen has read this material and is accurately reporting on what he read so far as he knows. How could Origen possibly have insisted in good faith that Josephus wrote a lot about James and attributed the sack of Jerusalem to James's persecution?
(1) Maybe Origen had a copy of the Antiquities that said so. Maybe, but all our received manuscripts show Josephus saying something very different and at generous length about why God foresook his Temple. Meanwhile, our Josephus has little to say about James except to mention his brother and that he lost his case. This alternative is only sparingly discussable.
(2) Maybe Origen was working from memory, and his memory let him down. This could be like when Jerome's memory failed him while discussing Josephus's version of the "Temple voices" incident (mentioned in the blog pieces already posted) or even more like when Bart Ehrman's memory led him to rewrite a chunk of the Pliny-Trajan correspondence.
Alternative (2) is both interesting and discussable. That's the one I pursued. As always under uncertainty, other scenarios are possible.