I love this analysis, it makes sense, i get people r annoyed with how heejae sees the manager in garam, however the way u explained how the manager leaving and heejae ( as Orpheus) failing to grasp onto him ( his Eurydice) makes total sense as there is now no more eurydice, however those memories will serve as a reminder for him to not repeat the same mistake in this “universe”
Though it's my opinion, I don't think the manager is the Eurydice here- I don't know how to say it, but isnt it all about music reaching the heart? Heejae truly cared about his manager because his manager was the only one who chose not to hate him at that time- after he left, it left Kim with a huge scar(because the number of people who truly cared for him and endured him was one), which he tried to cover by music, by introducing gems to the world. In fact, there isnt any Eurydice. imo, Eurydice signifies love itself. For a person who loves music more than anything, it's about whether they chose love or music.
That's also a possibility! Everything is speculative and theories right now. Heejae's internal narration in Ch.123 talks about creating gems, born out of a pure love for music but he also mentions, Perhaps hidden beneath his music his true mission is "to never lose you again." Many things could be taken from that chapter alone and it's what makes this series so good. I love when a story gets me analyzing.

I kind of enjoy how the glow of the dead manager shows up, it's fitting with the title of the series. We can picture Heejae as Orpheus and dead manager is Eurydice. In the myth, After not being able to bring back Eurydice from the dead/Underworld, she remains only as memories to Orpheus.