: The obvious flaw you have to address with this theory is exactly why no one
: mentions that there's another Ascendant Justice in the fleet around
: Unyielding Hierophant, and how it survives the destruction of the
: duplicate Ascendant Justice drawing the fleet into the Hierophant.

John mentions that after the Hierophant's explosion there are still a few ships left. As for why another Ascendant is never mentioned, that is more uncertain. I can buy Grizzlei's suggestion that Cortana didn't mention it to the humans because it didn't matter to the mission, but the Covenant don't mention it either. In the last chapter with the Prophet of Truth and Tartarus, reports are just coming in about the Hierophant and the "renegade flagship Ascendant Justice". Neither of them mention that there appears to be a second one somehow.

It could be possible that the original Justice wasn't at the Hierophant's fleet, but another Covenant ship there got the Cortana copy, which was then captured, made to work for the Covenant, and then given to the Ascendant Justice when it was ordered to head to Halo. But that's more speculative. A much simpler answer is that A Forerunner Crystal Did It. :p