How Could Egyptian Jewry Have Justified Setting Up a Rival Altar to the Temple of Jerusalem?

I principally write about Christian matters here at this blog. Yet as I have been delving a number of posts related to the existence of an Alexandrian Jewish house of worship at the turn of the Common Era my thoughts naturally turn to my ancestral religion.

Now let me explain that there is a very good reason why other people haven't written much on this subject - the rabbinic sources are refuted by the surviving texts of Josephus which report that the 'replica temple' actually was located in Heliopolis.

For those who prefer Josephus' report to the scores of rabbinic witnesses who say that this altar was in Alexandria, you will be able to still partake in this discussion - just substitute the word 'Heliopolis' for 'Alexandria.' The question still remains, how could Onias and those who followed him justified setting up a rival sacrificial cult to Jerusalem?

I think I have, with the help of my esteemed friend Professor Ruiaridh Boid, come up with a solution. Yet Professor Boid gave me a novel explanation where the editors of Josephus might have gotten the idea that the temple was actually in Heliopolis rather than Alexandria.

In all of the rabbinic narratives detailing the historical background to the establishment of the 'house of Onias' Isaiah chapter 19 is inevitably cited. It is here that the idea of 'the temple of Heliopolis' was born, for we read:

In that day five cities in Egypt will speak the language of Canaan and swear allegiance to the LORD Almighty. One of them will be called Heliopolis, the city of the sun. In that day there will be an altar to the LORD in the heart of Egypt, and a monument to the LORD at its border.

The editor of Josephus must have access to rabbinic references to the 'House of Onias' and the connection with Isaiah 19:18,19, which is where ultimately he got the wrong idea that as to the location of this place (as the specific location is unmentioned in the Mishnah).