I had an epiphany regarding the mythology a few days ago.
Previously, I'd been trying to tie the 'Underworld' references directly to the 'IT' of the Rift, to the thing we're looking for. However, I recently realized that the names of the ships destroyed during Salome's latest disappearance meant that this couldn't be the case.
Only human agents, operating at high levels within the Imperial hierarchy, are capable of arranging for the escort ships' names to be linked to the 'Seven Veils'/Ishtar mythology. These agents, this faction, are supportive of Salome and therefore are supportive of the quest to unlock the Rift mystery.
With that in mind, I believe the mythology should be considered to be referring NOT to the contents of the Rift, but instead, is a mechanism used by this faction to communicate with others, to draw people into the mystery and to allow them to recognize people who are interested in exposing the cover-up.
I think Salome has been accepted into the faction - receiving the bottle of Gerasian Liquor on Sol indicated that (Geras is the Greek god/personification of Old Age) - who subsequently rescued her when the Empire was going to lock her up. The ship names were a message to the outside world, saying "Don't worry, we've got her".
Somewhere in the Bubble (the Core) is a clue, not to the location of the Rift, but to the way to make contact with this hidden group - and that clue is buried in the mythology.
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