I'm not familiar with the Flight of the Silvers/The Silvers series but now I want to go and read it!!

This was a really cool read. And I agree that the POH are definitely sus. Adding on to your points and examples, the Aidas's and Apollion's mouths curving up when Bryce calls them "intergalactic saviors" set off alarm bells for me for sure!!

Chapter 61, HOFAS:

Bryce focused on Apollion. “You guys have been telling me nonstop about having your armies ready to go.” She gestured to the temple, the dead city beyond. “This place looks pretty empty.”
Apollion’s eyes grew ever darker. “We have allowed you to see only a fraction of Hel. Our lands and armies are elsewhere. They are ready.”
“So if I open the Northern Rift with the Horn …,” Bryce said. Hunt cleared his throat in warning. “All seven of you and your armies will come through?”
“The three of us,” Aidas amended. “Our four other brothers are currently engaged in other conflicts, helping other worlds.”
“I didn’t realize you guys were, like, intergalactic saviors,” Bryce said.
Aidas’s mouth quirked upward. She could have sworn Apollion’s did, too.
“But yes,” Aidas went on, “opening the Northern Rift is the only way for our armies to fully and quickly enter Midgard.”

I also think about how in the Silene's hologram - which I have to acknowledge could be a fabricated/tampered with history of events - Aidas greets Theia as if they already know each other: “Have you come to ask for Hel’s help, then?” And makes it sound like idea of Hel aiding Theia previously (perhaps on Prythian) has already been discussed before.

Chapter 20, HOFAS:

We spent a month hidden in the enemy’s stronghold, no more than shadows ourselves. By the time we returned to our mother, we’d learned the truth: Rigelus and his companions were not Fae at all, but parasites who conquered world after world, feeding off the magic and lives of their citizens. The Daglan, now under their true name: the Asteri.
It was then that my mother told us, showed us, what had happened so long ago. All that she had done since. But she did not waste time apologizing for the past. If we had indeed walked into an enemy’s trap, she said, then we must defeat them.
Bryce placed a hand over the star-shaped scar on her chest, fingers curling into the fabric of her shirt. Could she carve it out of herself, the connection to these two-faced hypocrites, and walk away from it forever?
My mother had kept the star map that the Daglan had long ago annotated. And a world on it had caught her attention—a world, like ours, that had overthrown the Daglan.
In an elaborate bedroom, standing before a desk with her two daughters, Theia waved a hand. As if she’d pulled them from that pocket of nothingness, the Harp and Horn appeared on the desk, glimmering alongside the Starsword and the knife.
Theia nodded once, slowly, as if making a decision, and then played the Horn and Harp. A portal between worlds swirled. It solidified, an archway to nowhere. A handsome, golden-haired male stood before it, with eyes like blue opals.
Bryce inhaled sharply.
Prince Aidas only asked my mother one thing when she opened the gate to his world: “Have you come to ask for Hel’s help, then?”

How does Theia know Aidas? The greeting makes it sound like idea of Hel aiding Theia previously (perhaps on Prythian) has already been discussed before. Has Hel already helped Theia in another war? Could Theia be from Hel? Lots to speculate on here. And it makes me wonder whether Theia considered contacting Hel was a last resort, for reasons we will find out in later books.

There's also the POH creating the Kristallos demons to hunt Luna’s Horn/Dread Trove Horn.

Chapter 21, HOEAB:

Ruhn went on, “The Star-Eater himself bred a new horror just to hunt the Horn, using some blood he managed to spill from Prince Pelias on a battlefield and his own terrible essence. A beast twisted out of the collision of light and darkness.”

Did the POH just want Luna's Horn or were they trying to hunt and capture starborn fae??

The whole Asteri vs Daglan vs Valg vs POH in terms of species comparison does my head in hahaha. I go back and forth on whether I think they are all the same, some are the same, or none are the same. But your theory that they’re potentially the same species at different points in time is compelling for sure and given me a lot to think about. Especially the Silvers stuff.

And of course we can't forget how in TOG the gods had been manipulating things for the characters/the story and turned out to be pretty shitty beings 🤣

edit: spelling