Behind me dips Eternity,
Mathematical term
"Term" means both a time period and the middle element in a mathematical equation. She's literally the variable between two infinities.
Directional death
East = sunrise = birth. Death moves east-to-west like the sun, dissolving as it approaches the actual west (eternity). She's reversing the expected direction.
Trinity language
"Son of none" contradicts Christian theology where Christ is Son of God. "Diversify / In duplicate divine" suggests the Trinity's three-in-one, but something's off about her phrasing.
Crescent geometry
A crescent has two points. She's the curved middle between two "midnights"—past and future both dark, unknowable. The sea image makes her position liquid, unstable.
Maelstrom above
Not in the sea where you'd expect it—in the sky. Heaven itself is chaotic, a whirlpool. This contradicts the "perfect pauseless monarchy" two stanzas earlier.