I read a comment that someone felt Hinterlands III dragged, whereas I just wanted it to keep going. I loved the story up until that point, but every Hinterlands section is just overflowing with symbolism and double-meanings. I look forward to reading what you've found if you end up posting about it here. I hope you end up playing the ruthless route if you haven't already, because everything you're saying about the beach seems to be tied to Asterion and the labyrinth itself, especially its inability to produce salt.
I've been thinking about the nature of the labyrinth and how it appears to people. There can't only be a couple dozen people in the world who are truly lost, so I wonder how much finding it relies on Hermes appearance. I say this because we see P and Storm end up finding the hotel without ever bumping into Hermes, yet P's grandfather clearly ran into Hermes before finding it. Yet as the achievement so aptly puts it, we've rewritten the story of Io and Argos, as well as coated Storm in a scent that Asterion would find familiar and meaningful. (As well as Poseidon.) How much of what happens in Chapter 18, including Hermes vanishing and his reaction to P and Storm, is because of their actions in the Hinterlands? There's so many connections and convergences to explore.
ETA: Thinking about the Argos and Io achievement is making me appreciate just how queer this story is. It doesn't just feature gay characters in what would normally be straight roles, it's about found family, making the best of your situation, and ultimately breaking tradition and what's been placed on you to find what you decide is happiness. I think that's half of why this story resonates so much with me. It's not just giving me something I can't find most places, it's commenting on it and using traditional myths and stories to create our own.