Ok so I finally got to watch it and here's a maybe obvious take: Hermes definitely knows. And I think it maybe lends credence to the "Orpheus knows" theory.
I guess we've always been aware because it's how Hermes opens the show with Road To Hell. But Road to Hell frames Hermes as a narrator breaking the third wall for the audience. Like sometimes he's "narrator" Hermes who's recalling the story to us, and sometimes he's "in-story" Hermes who's playing it out as it happened.
But at the end of the show, after Eurydice sinks and Orpheus collapses at the edge of the hole, Kurt Elling's Hermes turns his back to the audience and starts to sing Road to Hell Reprise directly! to! Orpheus!
Don't ask why, brother, don't ask how
He could have come so close
The song was written long ago
And that is how it goes
He's telling Orpheus that there was nothing he could have done, it was always going to play out this way. Because it already had before.
I've only seen clips from OBC and in one clip (I think previews) they staged it with Hermes standing centre downstage to sing the reprise to the audience because he was just narrating again. In later broadway clips, it starts with Hermes on the left of the hole and Orpheus on the right so they can be positioned opposite each other, but Hermes is still angled to the audience when he/she sings and is still framed as a narrator.
This version puts Orpheus closer to stage center behind the hole, so that it makes it clear that Hermes is actually talking to Orpheus and not us, because he has to turn his back mostly to the audience to face Orpheus.
It's a sad song
It's a sad tale
He speaks the last line like a parent teaching their child a life lesson. Kurt Elling's Hermes isn't an indifferent narrator sitting outside of the story. He's the same Hermes who's been helping and caring for Orpheus this whole time. He has a tissue in his hand and openly wipes his tears during the above lines (he holds it in the hand that's closer to the audience so he's not trying to hide it). (A tiktoker I follow said she's seen several casts in multiple countries and has never seen a Hermes cry).
The whole show he's been a father-figure to Orpheus, squeezing his shoulders in encouragement whenever Orpheus gets nervous. And the whole show, he's known that this tragedy was going to play out and he just has to let it happen. (I think the parental aspect is true of all of the productions, though some Hermes may be warmer than others -- I will mention that during Epic III, Kurt's Hermes looks like a nervous parent hovering to the side of their kid's recital and it's very endearing).
And then Hermes does eventually turn around to address the audience:
'Cause here’s the thing
To know how it ends
And still begin to sing it again
As if it might turn out this time
And then he turns back to face Orpheus one more time and gestures to him:
I learned that from a friend of mine
and Orpheus looks him in the eye and then gets up and goes back to his starting position from the beginning of the show.
So that he can try again.