My 2 cents.....

White Robes are explicitely mentioned to be NOT only for retired convocation members, they are also for certain other civil servants. In fact the seat of Elidibus is so special because it is the only seat which uses white robes when active.

I don't recall Hesperos mentioning he was doing this on Lahabrea's orders. He just said he would be 'more useful' to "him" as a hemitheos, but we actually do not know who this "Him" is.

The only smooth way I see for Themis to be Elidibus is for him to be incognito.
It makes absolutely no sense for Pandaemonium to happen BEFORE the MSQ when we pretty much have to traverse the zone and all the NPCs there have text from AFTER the MSQ. It would be really disjointed and I don't see SE doing that.
You could argue that they set it up with forcing us to go through the Ocular but imo that was just for the introduction cutscene so that Themis could tell us Azem foretold that he would meet 'a falling star' who would help him.
Now that I think of it the Themis/Elidibus post credit scene which teased pandaemonium made it really seem that it happened after the defeat of the Endsinger which would mean that Themis is a projection of the Elidibus caught in the Crystal Tower.
At this point it's more likely to me that the 'current' Lahabrea and Ascian Lahabrea are the same person.