(comment contains BIG spoilers)
Congratulations on getting the secret ending! I definitely made this one way to difficult to get and I'm making sure they're more approachable in the future. Someone has found the secret ending before (look up Ochre Secret Ending on YouTube) but you're the first, it seems, to want to really understand it!
As for the clue's wording, I tried to keep 'time' as the main object of the slipping and 'sand' the secondary object. Maybe brackets would have been better instead of commas:
"... to realise when time (like sand) is slipping through your fingers ..."
"it feels like time is slipping through my fingers"
There was originally no 'sand' there. But at some later point I added "like sand" to link it to an hourglass and must have missed that the sentence wasn't as clear anymore.
I'd love to answer your questions - I won't flat out spoil the whole story but I'll ask some questions (kinda like the Socratic method).
1. The elevator's functionality is clued by the inscription: it descends when four people stand around it who are ALL 'transferred' as you put it. The inscription is composed of three drawings. The first two depict an elevator descending when the humanoid figure has no swirl on their face. The third slice shows four dots around a (strangely familiar) square symbol - this is a bird's eye view of what is required. It wouldn't make sense if one person was living, because "Not a single soul had witnessed the opening of the studio. And not a single one ever would."
2. Olivia's face - like everyone else's - disappears after looking at her painting, but before her painting is placed back on the wall. When she's walking toward the obelisk, her face has already vanished. You say 'she's doomed to a different eternal life', but the normal ending and the secret are the EXACT SAME ending, from different perspectives. (Remember, she cannot avoid her fate, it's inescapable.) What part of Olivia do you think the normal ending focuses on? What about the secret ending? These two parts of her have been separated from one another.
3. The office-like chamber does represent something, but I don't think I'll reveal too much yet. It's related to an underlying theme. To explore the theme, consider:
a) Why does the symbol on the ticket (and the game icon) look the way it does? (it's not arbitrary)
b) What one feeling are all three characters suffering from (in different ways)? What are the sources of this feeling?
c) The oracle says "The combination of iron, the inorganic, and oxygen, the organic ... it's unnatural." They attribute it to iron-oxide in a literal sense, but what else is it reminiscent of metaphorically?
d) What was Olivia reminded of here: "Faceless people within square frames ... it reminds me of something but I can't quite put my finger on it."
e) The entire secret ending process places importance on 'taking a break'. From what? There's a literal answer - the game - but what's the metaphorical answer?
Your comment about the Oracle is spot on :)
And yeah, in retrospect it was definitely overly-obtuse. I think I got too adventurous after people found Pleonexia's secret ending so easily and ramped it up way too much. Your idea about locking the secret ending in when you enter the words is a good way to simplify a secret ending, and I'll consider doing something similar in the future. I might not change this one though, because I think it's nice every once in a while to have bits of my games that are extra hard to sus out, like rabbit holes, so that you can dig your fingers really deep into the them and still pull stuff out.
Thanks so much for playing!